Fix divider placement on reversed axes

[justin-sleep]

Fixes #4927. The divider placement algorithm now uses the correct boundary when the axis is reversed.

[archmoj]

@justin-sleep thanks very much for the PR.
Good catch!
Could you please add a mock for image test here: https://github.com/plotly/plotly.js/tree/master/test/image/mocks?

[justin-sleep]

Sure, those changes have been made.

[archmoj]

Sure, those changes have been made.

Awesome. Let's see the test results...

[archmoj]

Would you mind replacing the new baseline with the one below which is created in a docker?

Thank you!

[archmoj]

We need to figure out why few tests are failing.

[archmoj]

🎉 Great! LGTM.
Now that the CI as well as no_ci tests passed (on my machne), let me request a review from @alexcjohnson.

[alexcjohnson]

This fix in getDividerVals looks great. There's similar code in getBoundaryVals, does it need similar reversed-range handling or does it already work correctly? I've lost track of the distinction between these functions.

[archmoj]

This fix in getDividerVals looks great. There's similar code in getBoundaryVals, does it need similar reversed-range handling or does it already work correctly? I've lost track of the distinction between these functions.

It seems to me that a similar logic may be needed in getBoundaryVals.

[alexcjohnson]

If you can find a broken case with getBoundaryVals then sure - otherwise this PR looks great - thanks @justin-sleep ! 💃