Support decimal stepSize

[nagix]

This PR adds support for decimal stepSize for linear scales. In the current code,

• if stepSize >= 1, ticks will be rounded to integers
• if stepSize < 1, ticks will have the appropriate decimal precision
• If stepSize is not specified, the tick interval will be set to a 'nice' number, so there is no rounding issue

In this PR, the decimal precision is calculated based on stepSize if specified, or a generated 'nice' number or ticks.precision (only if stepSize is not specified).

Fixes #5392
Fixes #5579

[benmccann]

speified -> specified

[simonbrunel]

Maybe I didn't understand the issue / feature behind both options: I'm a bit worried about mixing both configs and have stepSize also controlling the precision when < 1 (overriding precision, which doesn't seem correct). Shouldn't we simply apply precision every time, not only when stepSize is undefined?

if stepSize < 1, ticks will have the appropriate decimal precision

I think we should support that case: stepSize: 0.524, precision: 2 (expecting 0, 1.05, 1.57, etc.)

if stepSize >= 1, ticks will be rounded to integers

What about: stepSize: 1.524, precision: 2? (expecting 0, 1.52, 3.05, etc.)

[nagix]

When stepSize is specified, we don’t need precision at all because stepSize can controll the precision. On the other hand, when stepSize is not specified but precision is specified, the tick interval is aligned with precision. To make things consistent, the same rule should be applied when both stepSize and precision are specified, but this may cause confusion. eg. stepSize: 0.524, precision: 2 results in 0, 0.52, 1.04, 1.56.

In that sense, it seems natual to me that precision is effective only when stepSize is not specified.

[nagix]

Or, should the auto-generated tick interval be completely independent from the precision? Should precision be only applied to the generated tick values?

In that case, there may be another confusion. eg. precision: 0 and the auto-generated tick interval: 1.5 results in 0, 2, 3, 5, 6.

[simonbrunel]

I understand, I thought that precision were used only on the displayed tick value (your previous comment), that's why I didn't understand why it's ignored when stepSize is not defined.

[nagix]

Thanks @benmccann, I have added a jsdoc and fixed a typo.

[simonbrunel]

... but this may cause confusion. eg. stepSize: 0.524, precision: 2 results in 0, 0.52, 1.04, 1.56

I think the confusion comes from the fact that the precision applies to two concepts: the tick generation and the displayed format. Initially introduced to fix #4103, it's definitely not to control the displayed format, so you right.

Or, should the auto-generated tick interval be completely independent from the precision? Should precision be only applied to the generated tick values?

I think it's possible to override the display format with scale.ticks.callback but that's not really convenient.

Thanks @nagix