Show value of each bar on top of it

[agzuniverse]

This change modifies the bar charts to show their values as a label on top of each bar if the user sets the new b.ShowLabel option to true.

This PR builds on #556
Closes #475

[agzuniverse]

@kortschak you mentioned #556 cannot be merged because it does not pass the CI build, so here's a PR which does.

[kortschak]

This needs a test.

[kortschak]

ShowValue?

[agzuniverse]

I've changed it to ShowValue.

[xiaofeng-zerone]

When can master branches use this feature?

[kortschak]

It would need to be merged first, and then it will be included in the next minor release. In order to be merged it needs to pass review, which will require that the change requests are satisfied.

[agzuniverse]

I'll look into adding a test for this soon.

[lynxplay]

Is there any plan on moving forward with this feature ?
It seems like a really nice addition and I'd personally love to use the master branch of gonum/plot rather than my own fork just for this feature.

[agzuniverse]

I think the only reason this PR wasn't merged is because I didn't write tests for this change. I thought of doing it but unfortunately couldn't find the time. @lynxplay if you could add a test the maintainers will probably merge this change in to master.

[lynxplay]

Yeah I tried looking into this but the generated plots don't respect the written values on top of the bars when cropping so one value is always cropped off.

Tests on this are mainly just a single generated graph and an image comparison no?

[sbinet]

yes, we like to have a simple testdata/foo_golden.{png,svg,...} reference file and a test (or an example) generating that file to compare with and make sure we don't introduce regressions when refactoring code.

here is an example:

• https://github.com/gonum/plot/blob/v0.9.0/plotter/barchart_example_test.go#L159 (example generating testdata/barChart_positiveNegative.png)
• https://github.com/gonum/plot/blob/v0.9.0/plotter/barchart_test.go#L19 (the test that runs ExampleBarChart_positiveNegative and leverages the cmpimg package that compares with reference files)
• https://github.com/gonum/plot/blob/v0.9.0/plotter/testdata/barChart_positiveNegative_golden.png (the reference file)

(with my apologies for the belated answer.)