feature: export chart to data url

[eric-gitta-moore]

What kind of change does this PR introduce?

• [ ] Bugfix
• [x] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

What is the current behavior?

Unable to export chart. #1906

What is the new behavior?

The method of exporting data-url is added for BaseChart, and users can download images by obtaining data-url and a tag.

export options

export to svg

export to bitmap

with transparent

Does this PR introduce a breaking change? (check one with "x")

• [ ] Yes
• [x] No

[Lyq322]

In the image for combo chart the legend is cut off for all three formats

[Lyq322]

The image for advanced pie chart is also cut off in many places.

[kedij777]

Incredible feature! But it seems like the exported image won't include legend if I choose legend at the bottom.

[sudarshansb143]

Why is this feature is still not available on demo site ?

[eric-gitta-moore]

@sudarshansb143 Thank you for your attention, here are two reasons that may cause temporarily unavailable reasons.

1. There are also problems with exporting individual charts
2. Project merging code may not be very active