Enable a legend display for charts

I often track fairly small chunks of time (<15mins), which means my week/day charts end up with a lot of blocks that just have one or two letters, making it hard to determine what the time was actually spent doing.

It would be good if I could enable a legend so that the colours used for these blocks did have their full tag spelled out somewhere below the chart.

No file, test, or entry point is named. Start by locating the C++ CLI chart/reporting entry point and its existing chart output; then determine how an optional legend can be enabled below week/day charts. Done means the legend spells out the full tags for the displayed colours while existing output remains unchanged when it is not enabled.

Written by the indexing model from the issue text.