D3 Programmatic rotation of radial Graph

Question

I have a graph that uses d3.radialLine that can be seen in this fiddle. My data points on the line are visualized by plotting the circle on the proper axis value. I need to have a line down the middle of the svg element that will give me the reading of the plotted data point that is touching it. You can see a graphic image of what I'm trying to accomplish:

I would like to programmatically rotate, for example clockwise so that December will be inline with the red line, and be able to read that data point. I know that I'll have to get the x2 value of the red line and use 'x2' of the December value to find out how many degrees I would need to rotate the graph, but I can't seem to figure that part out. The code was taken from Radar Chart.

I know that formula for rotating..d3.select(...).attr('transform', 'rotate(degree, x, y)'

Code

            ////////////////////////////////////////////////////////////// 
            //////////////////////// Set-Up ////////////////////////////// 
            ////////////////////////////////////////////////////////////// 

            var margin = {top: 100, right: 100, bottom: 100, left: 100},
                width = Math.min(700, window.innerWidth - 10) - margin.left - margin.right,
                height = Math.min(width, window.innerHeight - margin.top - margin.bottom - 20);

            ////////////////////////////////////////////////////////////// 
            ////////////////////////// Data ////////////////////////////// 
            ////////////////////////////////////////////////////////////// 

            var data = [
                      // Yearly
                      [{axis:"Jan",value: 700},
                      {axis:"Feb",value: 1453},
                      {axis:"March",value: 1300},
                      {axis:"April",value: 1534},
                      {axis:"May",value: 1534},
                      {axis:"June",value: 1547},
                      {axis:"July",value: 1100},
                      {axis:"August",value: 1800},
                      {axis:"September",value: 1700},
                      {axis:"October",value: 1500},
                      {axis:"November",value: 1000},
                      {axis:"December",value: 1200}
                      ]
                    ];

            ////////////////////////////////////////////////////////////// 
            //////////////////// Draw the Chart ////////////////////////// 
            ////////////////////////////////////////////////////////////// 

            var color = d3.scaleOrdinal()
                .range(["#58D6C7","#CC333F","#00A0B0"]);

            var radarChartOptions = {
              w: width,
              h: height,
              margin: margin,
              maxValue: 0.5,
              levels: 5,
              roundStrokes: true,
              color: color,
              opacityCircles: 0.1
            };


            //Call function to draw the Radar chart
            RadarChart(".radarChart", data, radarChartOptions);


      /////////////////////////////////////////////////////////
/////////////// The Radar Chart Function ////////////////
/////////////// Written by Nadieh Bremer ////////////////
////////////////// VisualCinnamon.com ///////////////////
/////////// Inspired by the code of alangrafu ///////////
/////////////////////////////////////////////////////////

function RadarChart(id, data, options) {
    var cfg = {
     w: 600,                //Width of the circle
     h: 600,                //Height of the circle
     margin: {top: 20, right: 20, bottom: 20, left: 20}, //The margins of the SVG
     levels: 3,             //How many levels or inner circles should there be drawn
     maxValue: 0,           //What is the value that the biggest circle will represent
     labelFactor: 1.25,     //How much farther than the radius of the outer circle should the labels be placed
     wrapWidth: 60,         //The number of pixels after which a label needs to be given a new line
     opacityArea: 0.35,     //The opacity of the area of the blob
     dotRadius: 4,          //The size of the colored circles of each blog
     opacityCircles: 0.1,   //The opacity of the circles of each blob
     strokeWidth: 2,        //The width of the stroke around each blob
     roundStrokes: false,   //If true the area and stroke will follow a round path (cardinal-closed)
     color: d3.scaleOrdinal(d3.schemeCategory10)    //Color function
    };

    //Put all of the options into a variable called cfg
    if('undefined' !== typeof options){
      for(var i in options){
        if('undefined' !== typeof options[i]){ cfg[i] = options[i]; }
      }//for i
    }//if

    //If the supplied maxValue is smaller than the actual one, replace by the max in the data
    var maxValue = Math.max(cfg.maxValue, d3.max(data, function(i){return d3.max(i.map(function(o){return o.value;}))}));

    var allAxis = (data[0].map(function(i, j){return i.axis})), //Names of each axis
        total = allAxis.length,                 //The number of different axes
        radius = Math.min(cfg.w/2, cfg.h/2),    //Radius of the outermost circle
        Format = d3.format(''),             //Percentage formatting
        angleSlice = Math.PI * 2 / total;       //The width in radians of each "slice"

    //Scale for the radius
    var rScale = d3.scaleLinear()
        .range([0, radius])
        .domain([0, maxValue]);

    /////////////////////////////////////////////////////////
    //////////// Create the container SVG and g /////////////
    /////////////////////////////////////////////////////////

    //Remove whatever chart with the same id/class was present before
    d3.select(id).select("svg").remove();

    //Initiate the radar chart SVG
    var svg = d3.select(id).append("svg")
            .attr("width",  cfg.w + cfg.margin.left + cfg.margin.right)
            .attr("height", cfg.h + cfg.margin.top + cfg.margin.bottom)
            .attr("class", "radar"+id);
    //Append a g element        
    var g = svg.append("g")
            .attr("transform", "translate(" + (cfg.w/2 + cfg.margin.left) + "," + (cfg.h/2 + cfg.margin.top) + ")");

    /////////////////////////////////////////////////////////
    ////////// Glow filter for some extra pizzazz ///////////
    /////////////////////////////////////////////////////////

    //Filter for the outside glow
    var filter = g.append('defs').append('filter').attr('id','glow'),
        feGaussianBlur = filter.append('feGaussianBlur').attr('stdDeviation','2.5').attr('result','coloredBlur'),
        feMerge = filter.append('feMerge'),
        feMergeNode_1 = feMerge.append('feMergeNode').attr('in','coloredBlur'),
        feMergeNode_2 = feMerge.append('feMergeNode').attr('in','SourceGraphic');

    /////////////////////////////////////////////////////////
    /////////////// Draw the Circular grid //////////////////
    /////////////////////////////////////////////////////////

    //Wrapper for the grid & axes
    var axisGrid = g.append("g").attr("class", "axisWrapper");

    //Draw the background circles
    axisGrid.selectAll(".levels")
       .data(d3.range(1,(cfg.levels+1)).reverse())
       .enter()
        .append("circle")
        .attr("class", "gridCircle")
        .attr("r", function(d, i){return radius/cfg.levels*d;})
        .style("fill", "#CDCDCD")
        .style("stroke", "#CDCDCD")
        .style("fill-opacity", cfg.opacityCircles)
        .style("filter" , "url(#glow)");

    //Text indicating at what % each level is
    axisGrid.selectAll(".axisLabel")
       .data(d3.range(1,(cfg.levels+1)).reverse())
       .enter().append("text")
       .attr("class", "axisLabel")
       .attr("x", 4)
       .attr("y", function(d){return -d*radius/cfg.levels;})
       .attr("dy", "0.4em")
       .style("font-size", "10px")
       .attr("fill", "#737373")
       .text(function(d,i) { return Format(maxValue * d/cfg.levels); });

    /////////////////////////////////////////////////////////
    //////////////////// Draw the axes //////////////////////
    /////////////////////////////////////////////////////////

    //Create the straight lines radiating outward from the center
    var axis = axisGrid.selectAll(".axis")
        .data(allAxis)
        .enter()
        .append("g")
        .attr("class", "axis");
    //Append the lines
    axis.append("line")
        .attr("x1", 0)
        .attr("y1", 0)
        .attr("x2", function(d, i){ return rScale(maxValue*1.1) * Math.cos(angleSlice*i - Math.PI/2); })
        .attr("y2", function(d, i){ return rScale(maxValue*1.1) * Math.sin(angleSlice*i - Math.PI/2); })
        .attr("class", "line")
    .style("stroke", "white")
        .style("stroke-width", "2px");

    //Append the labels at each axis
    axis.append("text")
        .attr("class", "legend")
        .style("font-size", "11px")
        .attr("text-anchor", "middle")
        .attr("dy", "0.35em")
        .attr("x", function(d, i){ return rScale(maxValue * cfg.labelFactor) * Math.cos(angleSlice*i - Math.PI/2); })
        .attr("y", function(d, i){ return rScale(maxValue * cfg.labelFactor) * Math.sin(angleSlice*i - Math.PI/2); })
        .text(function(d){return d})
        .call(wrap, cfg.wrapWidth);

    /////////////////////////////////////////////////////////
    ///////////// Draw the radar chart blobs ////////////////
    /////////////////////////////////////////////////////////

    //The radial line function
    var radarLine = d3.radialLine()
        .curve(d3.curveLinearClosed)
        .radius(function(d) { return rScale(d.value); })
        .angle(function(d,i) {  return i*angleSlice; });

    if(cfg.roundStrokes) {
        radarLine.curve(d3.curveCardinalClosed);
    }

    //Create a wrapper for the blobs    
    var blobWrapper = g.selectAll(".radarWrapper")
        .data(data)
        .enter().append("g")
        .attr("class", "radarWrapper");



    //Append the backgrounds    
    blobWrapper
        .append("path")
        .attr("class", "radarArea")
        .attr("d", function(d,i) { return radarLine(d); })
        .style("fill", function(d,i) { return cfg.color(i); })
        .style("fill-opacity", cfg.opacityArea)
        .on('mouseover', function (d,i){
            //Dim all blobs
            d3.selectAll(".radarArea")
                .transition().duration(200)
                .style("fill-opacity", 0.1); 
            //Bring back the hovered over blob
            d3.select(this)
                .transition().duration(200)
                .style("fill-opacity", 0.7);    
        })
        .on('mouseout', function(){
            //Bring back all blobs
            d3.selectAll(".radarArea")
                .transition().duration(200)
                .style("fill-opacity", cfg.opacityArea);
        });

    //Create the outlines   
    blobWrapper.append("path")
        .attr("class", "radarStroke")
        .attr("d", function(d,i) { return radarLine(d); })
        .style("stroke-width", cfg.strokeWidth + "px")
        .style("stroke", function(d,i) { return cfg.color(i); })
        .style("fill", "none")
        .style("filter" , "url(#glow)");        

    //Append the circles
    blobWrapper.selectAll(".radarCircle")
        .data(function(d,i) { return d; })
        .enter().append("circle")
        .attr("class", "radarCircle")
        .attr("r", cfg.dotRadius)
        .attr("cx", function(d,i){ return rScale(d.value) * Math.cos(angleSlice*i - Math.PI/2); })
        .attr("cy", function(d,i){ return rScale(d.value) * Math.sin(angleSlice*i - Math.PI/2); })
        .style("fill", function(d,i,j) { return cfg.color(j); })
        .style("fill-opacity", 0.8);

    /////////////////////////////////////////////////////////
    //////// Append invisible circles for tooltip ///////////
    /////////////////////////////////////////////////////////

    //Wrapper for the invisible circles on top
    var blobCircleWrapper = g.selectAll(".radarCircleWrapper")
        .data(data)
        .enter().append("g")
        .attr("class", "radarCircleWrapper");

    //Append a set of invisible circles on top for the mouseover pop-up
    blobCircleWrapper.selectAll(".radarInvisibleCircle")
        .data(function(d,i) { return d; })
        .enter().append("circle")
        .attr("class", "radarInvisibleCircle")
        .attr("r", cfg.dotRadius*1.5)
        .attr("cx", function(d,i){ return rScale(d.value) * Math.cos(angleSlice*i - Math.PI/2); })
        .attr("cy", function(d,i){ return rScale(d.value) * Math.sin(angleSlice*i - Math.PI/2); })
        .style("fill", "none")
        .style("pointer-events", "all")
        .on("mouseover", function(d,i) {
            newX =  parseFloat(d3.select(this).attr('cx')) - 10;
            newY =  parseFloat(d3.select(this).attr('cy')) - 10;

            tooltip
                .attr('x', newX)
                .attr('y', newY)
                .text(Format(d.value))
                .transition().duration(200)
                .style('opacity', 1);
        })
        .on("mouseout", function(){
            tooltip.transition().duration(200)
                .style("opacity", 0);
        });

    //Set up the small tooltip for when you hover over a circle
    var tooltip = g.append("text")
        .attr("class", "tooltip")
        .style("opacity", 0);

    /////////////////////////////////////////////////////////
    /////////////////// Helper Function /////////////////////
    /////////////////////////////////////////////////////////

    //Taken from http://bl.ocks.org/mbostock/7555321
    //Wraps SVG text    
    function wrap(text, width) {
      text.each(function() {
        var text = d3.select(this),
            words = text.text().split(/\s+/).reverse(),
            word,
            line = [],
            lineNumber = 0,
            lineHeight = 1.4, // ems
            y = text.attr("y"),
            x = text.attr("x"),
            dy = parseFloat(text.attr("dy")),
            tspan = text.text(null).append("tspan").attr("x", x).attr("y", y).attr("dy", dy + "em");

        while (word = words.pop()) {
          line.push(word);
          tspan.text(line.join(" "));
          if (tspan.node().getComputedTextLength() > width) {
            line.pop();
            tspan.text(line.join(" "));
            line = [word];
            tspan = text.append("tspan").attr("x", x).attr("y", y).attr("dy", ++lineNumber * lineHeight + dy + "em").text(word);
          }
        }
      });
    }//wrap 

}//RadarChart

Answer 1

For rotating you just need to increase your initial angle offset over time.

Here's an example:

https://bl.ocks.org/tezzutezzu/c9d8706587e8f5b5d72084b083b502f8

As we know the angle is dependent to the index of the datum, you can calculate the value of the red dots under the red line in this way.

var currentAngle = (offset - Math.PI/2) % (Math.PI*2);
var currentIndex = Math.floor( (currentAngle/(Math.PI*2)) * data[0].length);
var currentValue = data[0][currentIndex].value;