Simple Pie Chart

A pie chart represents numerical proportions as slices of a pie. The size of a slice is proportional to the quantity it represents. Each slice usually has a different color.

NOTE: It is often better to use bar charts instead of pie charts, for example because comparing the sizes of two circular sectors in a pie chart can be more challenging than comparing the sizes of two bars in a bar chart.

Let's study this potential problem in a future activity. For now, let's implement a function to create simple pie charts!

Interface

Our function has the following signature:

def pie_chart(
  values: list[float], colors: list[Color], radius: float
) -> Graphic:

It takes the following parameters:

• values -- the list of numerical values to plot
• colors -- the list of colors to use for plotting each value
• radius -- the radius of the pie chart

In the end, you should be able to call it like this:

show_graphic(pie_chart([1.0, 2.0, 3.0], [red, green, blue], 100))

This call should produce the following chart:

Decomposition

A pie chart is made up of slices, one slice per value. Each slice is a circular sector.

Implementation

You can use the circular_sector function to create a sector.

Check out the API (by hovering over the red word) to figure out where the pinning position of a circular sector is located. Use that fact for rotating and composing multiple circular sectors into a pie.

Better Colors and More Slices

Now let's use your function for a slightly more complex pie chart. To be able to clearly distinguish the different pieces in a categorical visualizations (e.g., the different slices in a pie chart), picking a set of colors that are easily distinguishable is a good idea. The Color Brewer is a great resource to create such color palettes. Here we include a palette that is optimized for visualizing 10 different categories.

Note that in the code below, the number of values and the number of colors differ. Your pie_chart function should be able to deal with this situation. It should cycle through the given number of colors: If there are fewer values than colors, it should use the first N colors. If there are more values than colors, it should cycle through colors reusing the same color for multiple slices.

What You Learned

You learned how to compose multiple circular sectors into a pie chart. You also learned about good color palettes for qualitative / categorical visualizations.