Bar chart support - #323

I noticed that it was originally planned to support bar charts, but that feature wasn't implemented. I added the command csvtk plot bar that allows you to output a bar chart.

The PR is still missing documentation and examples. I was testing it with my own data, and it almost works as expected. I need to make sure labels and legend are set correctly and aligned to the bars in all scenarios (e.g. grouped data).

I wanted to first open the PR to see if this is something you'd be willing to include in csvtk so to know if it's worth finalizing the code and documentation.

For instance, running this:

cat testdata/date2value.csv \
    | csvtk gather -f 2- -k type -v count \
    | csvtk plot bar --group-field type -x date -y count --legend-top \
    > bar-chart.png

Creates this graph:

Let me know what you think 🙂