bm_chart: Render line, bar and pie charts from data values

This class can be used to render several types of charts from data sets.

It can create an image and render individual data values that appear in the chart image as lines, bars and pie slices.

Each value can be rendered with a text label. The value colors and margin distances are configurable.

The generated chart images can be saved to an image file in the PNG format or served as the current script output.