Add way for toImage to export images with current graph width/height

[etpinard]

resolves #3743 ... in a slightly more general way than proposed in that issue.

This PR adds special logic to Plotly.toImage (which is called by Plotly.downloadImage, which is itself called when clicking on the toImage modebar button) for options width:null and height:null.

In brief, Plotly.toImage(gd, {width:null, height:null}) will use the width/height values found in gd._fullLayout to generate its output image. Equivalently,

Plotly.react(gd, data, layout, {
  toImageButtonOptions: {width: null, height: null}
})

and clicking on the toImage modebar button will generate an image using the current gd._fullLayout.width and gd._fullLayout.height values - which (should at least) always store the current graph width/height values.

@plotly/plotly_js @nicolaskruchten - let me know what you think!

[nicolaskruchten]

Hah that’s actually what I had initially tried and thought might work so I’m totally down with that approach. /cc @chriddyp

[chriddyp]

lgtm!

[antoinerg]

Beautiful feature @etpinard !

Could config options toImageButtonOptions: {width: null, height: null} be the default when responsive: true?

[etpinard]

Could config options toImageButtonOptions: {width: null, height: null} be the default when responsive: true?

I'd call that a breaking change. For sure in v2 though.

[antoinerg]

I'd call that a breaking change. For sure in v2 though.

Ok, then the only request I have is to add it to the v2 wishlist!

💃

[etpinard]

Ok, then the only request I have is to add it to the v2 wishlist!

✏️ in #1576 (comment) - for better context info.