Plotting 2D heat map

Question

I have a chart on which I want to plot a heat map; the only data I have is humidity and temperature, which represent a point in the chart.

How do I get the rectangular type of heat map on the chart in c#?

What I want is similar to picture below :

What I really want is a rectangular region in the chart which is plotted in different color based on the point that i get from the list of points and form the colorful section in the chart.

TaW · Accepted Answer

You have a choice of at least three ways to create a chart with colored rectangles that make up a heat map.

Here is one example that uses/abuses a DataGridView. While I would not suggest this, the post contains a useful function that creates nice color lists to use in your task.

Then there is the option to draw the chart using GDI+ methods, namely Graphics.FillRectangle. This not hard at all but once you want to get those nice extras a Chart control offers, like scaling, axes, tooltips etc the work adds up.. See below!

So let's have a look at option three: Using the Chart control from the DataVisualization namespace.

Let's first assume that you have created a list of colors:

List colorList = new List();

And that you have managed to project your data onto a 2D array of int indices that point into the color list:

int[,] coloredData = null;

Next you have to pick a ChartType for your Series S1 There really is only one I can think of that will help:

S1.ChartType = SeriesChartType.Point;

Points are displayed by Markers. We want the DataPoints not really displayed as one of the standard MarkerTypes.

Square would be ok, if we wanted to display squares; but for rectangles it will not work well: Even if we let them overlap there will still be points at the borders that have a different size because they don't fully overlap..

So we use a custom marker by setting the MarkerImage of each point to a bitmap of a suitable size and color.

Here is a loop that adds the DataPoints to our Series and sets each to have a MarkerImage:

for (int x = 1; x < coloredData.GetLength(0); x++)
    for (int y = 1; y < coloredData.GetLength(1); y++)
    {
        int pt = S1.Points.AddXY(x, y);
        S1.Points[pt].MarkerImage = "NI" +  coloredData[x,y];

    }

This takes some explaining: To set a MarkerImage that is not at a path on the disk, it has to reside in the Chart's Images collection. This means is needs to be of type NamedImage. Any image will do, but it has to have a unique name string added to identify it in the NamedImagesCollection . I chose the names to be 'NI1', 'NI2'..

Obviously we need to create all those images; here is a function to do that:

void createMarkers(Chart chart, int count)
{
    // rough calculation:
    int sw = chart.ClientSize.Width / coloredData.GetLength(0);
    int sh = chart.ClientSize.Height / coloredData.GetLength(1);

    // clean up previous images:
    foreach(NamedImage ni in chart1.Images) ni.Dispose();
    chart.Images.Clear();

    // now create count images:
    for (int i = 0; i < count; i++)
    {
        Bitmap bmp = new Bitmap(sw, sh);
        using (Graphics G = Graphics.FromImage(bmp))
            G.Clear(colorList[i]);
        chart.Images.Add(new NamedImage("NI" + i, bmp));
    }
}

We want all markers to have at least roughly the right size; so whenever that size changes we set it again:

void setMarkerSize(Chart chart)
{
    int sx = chart1.ClientSize.Width / coloredData.GetLength(0);
    int sy = chart1.ClientSize.Height / coloredData.GetLength(1);
    chart1.Series["S1"].MarkerSize = (int)Math.Max(sx, sy);
}

This doesn't care much about details like the InnerPlotPosition, i.e. the actual area to draw to; so here is some room for refinement..!

We call this when we set up the chart but also upon resizing:

private void chart1_Resize(object sender, EventArgs e)
{
    setMarkerSize(chart1);
    createMarkers(chart1, 100);
}

Let's have a look at the result using some cheap testdata:

As you can see resizing works ok..

Here is the full code that set up my example:

private void button6_Click(object sender, EventArgs e)
{
    List stopColors = new List()
    { Color.Blue, Color.Cyan, Color.YellowGreen, Color.Orange, Color.Red };
    colorList = interpolateColors(stopColors, 100);

    coloredData = getCData(32, 24);
    // basic setup..
    chart1.ChartAreas.Clear();
    ChartArea CA = chart1.ChartAreas.Add("CA");
    chart1.Series.Clear();
    Series S1 = chart1.Series.Add("S1");
    chart1.Legends.Clear();
    // we choose a charttype that lets us add points freely:
    S1.ChartType = SeriesChartType.Point;

    Size sz = chart1.ClientSize;

    // we need to make the markers large enough to fill the area completely:
    setMarkerSize(chart1);
    createMarkers(chart1, 100);

    // now we fill in the datapoints
    for (int x = 1; x < coloredData.GetLength(0); x++)
        for (int y = 1; y < coloredData.GetLength(1); y++)
        {
            int pt = S1.Points.AddXY(x, y);
            //  S1.Points[pt].Color = coloredData[x, y];

            S1.Points[pt].MarkerImage = "NI" +  coloredData[x,y];
        }
}

A few notes on limitations:

The point will always sit on top of any gridlines. If you really needs those you will have to draw them on top in one of the the Paint events.
The labels as shown are referring to the integers indices of the data array. If you want to show the original data, one way would be to add CustomLabels to the axes.. See here for an example!

This should give you an idea of what you can do with a Chart control; to complete your confusion here is how to draw those rectangles in GDI+ using the same colors and data:

Bitmap getChartImg(float[,] data, Size sz, Padding pad) 
{
    Bitmap bmp = new Bitmap(sz.Width , sz.Height);
    using (Graphics G = Graphics.FromImage(bmp))
    {
        float w = 1f * (sz.Width - pad.Left - pad.Right) / coloredData.GetLength(0);
        float h = 1f * (sz.Height - pad.Top - pad.Bottom) / coloredData.GetLength(1);
        for (int x = 0; x < coloredData.GetLength(0); x++)
            for (int y = 0; y < coloredData.GetLength(1); y++)
            {
                using (SolidBrush brush = new SolidBrush(colorList[coloredData[x,y]]))
                    G.FillRectangle(brush, pad.Left + x * w, y * h - pad.Bottom, w, h);
            }

    }
    return bmp;
}

The resulting Bitmap looks familiar:

That was simple; but to add all the extras into the space reserved by the padding will not be so easy..

Plotting 2D heat map

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