How to change color of drawn points on pyGal line chart

Question

I am creating a line graph using pygal by passing in an array of numbers to be graphed. I am wishing for the points marked on the graph to change color when they are in/outside of a certain range. I.e. If there is a point logged over 40, color it red, if there is a point logged under 20, color it blue.

There does not seem to be an easy way to loop through the array and draw a single point.

The graph is being made with the following code:

    customStyle = Style(colors=["#000000"])
    chart = pygal.Line(style=customStyle)
    chart.title = 'Browser usage evolution (in %)'
    chart.x_labels = recordedDates
    chart.add('Humidity', recordedHumidity)
    chart.render_to_png("out.png")

I would like to have all points above 40 red and below 20 blue.

Answer 1

You can replace a number in the array with a dict that tells Pygal how to render the data point. This dict must contain the key value, which is the number you would have passed, alongside any customisation options you want to use. The list of available options is provided on the value configuration page of the docs, but the one you need here is color.

You can simply iterate over your existing array, creating a dictionary where color is set appropriately for the value:

data = []
for v in recordedHumidity:
    if v > 40:
        data.append({"value": v, "color": "red"})
    elif v < 20:
        data.append({"value": v, "color": "blue"})
    else:
        data.append(v)

You can then pass the newly created array when adding the series:

customStyle = Style(colors=["#000000"])
chart = pygal.Line(style=customStyle)
chart.x_labels = recordedDates
chart.add('Humidity', data)
chart.render_to_png("out.png")

You might also want to look at the chart configuration and series configuration pages in the docs to see how to customise other aspects of the chart, such as the size of the markers.