Needs tests

[chrispahm]

Re-occuring issues as https://github.com/chrispahm/chartjs-plugin-dragdata/issues/39 show that this plugin is in need of some well defined tests.

The tests should cover

• dragging on all types of charts is working
• dragging on stacked charts is working
• dragging to 0 is working
• the callbacks are provided with the correct function arguments
• rounding of values is working correct
• dragData: false within a dataset is working as expected
• touch device support

Did I forget something? I will try my best to review open PRs, and get started with writing these tests in the coming weeks. However, help would be greatly appreciated!

[artus9033]

Since the tests introduced in #132, #97, #120, #140 and the coverage level reported in codecov is $\approx 90\%$, this issue can now be closed 🎉