Legend Symbol Parameter to override point style

[touletan]

I would like to include new options to change the symbol used on chart Legend. I also want this to be independant from the pointStyle value (e.g we can keep 'circle' as line point, but use 'line' as legend symbol).

Potential solution:
add legendSymbol value at options/legend/labels and datasets levels.

legend selection priority selection

1. If usePointStyle = true -> use pointStyle value (circle by default) for legend symbol
2. if legendSymbol exists at dataset level -> use that value
3. if legendSymbol exists at /legend/labels level -> use that value
4. Default value: rectLg
   Available value: same as pointStyle + rectLg, lineLg and rectRoundedLg.
   *Lg symbols are usign boxWidth for Width while others are usign fontSize * Math.SQRT2

proposed implementation:
https://github.com/touletan/Chart.js/

[etimberg]

@touletan this looks like an interesting idea. I'd be happy to review these changes in a PR.

[touletan]

done...not familiar with that process, so it may not be good at first time.

[etimberg]

This can be done using a combination of the usePointStyle and generateLabels options. See https://jsfiddle.net/tg2qsfx7/

@kurkle do you think it's worth adding a specific option for this? It seems to be commonly requested (see #4727). On one hand I can see it being useful, but there's a nice workaround using the above 2 options that doesn't increase our shipped size.