added custom legend to heatmap chart

[igorkasyanchuk]

Explanation About What Code Achieves:

Ability to specify the custom label on the heatmap.

Screenshots/GIFs:

http://prntscr.com/hvy8xu

Steps To Test:

Create a heatmap chart and pass

labels: ["Jan (8)", "Feb (2)", "Mar (5)", "Apr (1)", "May (8)", "Jun (8)", "Jul (7)", "Aug (0)", "Sep (7)", "Oct (1)", "Nov (2)", "December (9)"],

TODOs:

• None

[pratu16x7]

@igorkasyanchuk Thanks for contributing!
It seems that rather than passing hardcoded labels, the label numbers should rather be calculated from the total values in that month. Such a feature would then be togglable via chart options, with property (named maybe showDomainTotals ;)