[Feature] supports action "saveAsImage"

[TonyYanOnFire]

What problem does this feature solve?

Add an action "saveAsImage" for dispatchAction so that you can write code to trigger the saveAsImage event of an echart instance.

What does the proposed API look like?

myChart.dispatchAction({
    type: 'saveAsImage',
    // saveAsImage options
    ...
})

[Ovilia]

Can this be done by chart.getDom().toDataURL()?

[TonyYanOnFire]

You sure can use current APIs like getDataURL and getConnectedDataURL to extract the url and save it as an image, which is exactly what I'm doing in my production codes right now. But from what I see there are at least 2 inconveniences:

1. I have to somehow extract and maintain the option I use both in defining toolbox.feature.saveAsImage and getDataURL
2. I have to define my own download method, which has a great chance to have less compatibility compare to the one echarts has already defined

What I wanna do is as simply as trigger the saveAsImage action, and at first I was like "How hard it is? We already have a saveAasImage button here!" . But turns out the work I have to put into is not that simple, to be fair.

[TonyYanOnFire]

Hey, anyone still interested in this feature? If no official team members reply within 24 hours, I'm gonna close this issue and related PR.