per cube histogram gets calculated when switching to a new Stokes value

[kswang1029]

Describe the bug
The render configuration widget and the contour rendering dialog have a histogram plot showing the per-channel histogram by default. Optionally users may switch to per-cube mode to see a histogram derived using the entire cube.

When a 4D cube with multiple Stokes (eg Stokes IQUV) is loaded in CARTA and we request to see the per-cube histogram of Stokes I, we will see the backend starts to deliver partial per-cube histogram to the frontend. Once it is done, if we use the animator to switch to Stokes Q, for example, we will see the "Histogram" dropdown switches to per-channel (as expected) but the backend starts to calculate the per-cube histogram of Stokes Q and once it is finished, the backend then calculates the per-channel histogram of the Stokes Q in the image viewer.

To Reproduce
Steps to reproduce the behavior:

1. load a 4D cube with multiple Stokes and enable the JS console
2. request per-cube histogram
3. use the animator to switch to Stokes Q and monitor the JS console
4. we should see per-cube histogram being calculated first, then the per-channel histogram is calculated.

When the per-cube histogram is calculated we will see nothing happens from the GUI. But if we request a per-cube histogram when the backend is busy on calculating the unexpected per-cube histogram, we will see the progress bar shows up repeatedly. Sometimes this results in incorrect clip bounds (I saw 0,1).

Expected behavior
The expected behavior in step 4 should be that only the per-channel histogram is calculated. I guess the root cause is the logic behind SET_IMAGE_CHANNELS (where we can set a Stokes value via stokes parameter) and SET_HISTOGRAM_REQUIREMENTS (where we can request a per-channel or a per-cube histogram via region_id parameter)

Screenshots or videos
This is a screenshot of the JS console. The last REGION_HISTOGRAM_DATA carries the per-channel histogram. All the other REGION_HISTOGRAM_DATA before carry the unnecessary per-cube histogram data.

Platform info (please complete the following information):

• OS: only tested with macOS Monterey
• Browser: chrome
• Version 97.0.4692.71

Additional context
N/A

[markccchiang]

@kswang1029 In the contour rendering dialog, if we select per-cube histogram and then switch to another Stokes. The default is still per-cube histogram for another Stokes, not per-channel histogram, this behavior is not the same as that in the render configuration widget. Should it be changed? If so, I guess the contour rendering dialog in the frontend also needs to be fixed. The default histogram for the new Stokes is always per-channel histogram.

[kswang1029]

@markccchiang Good point. Could you check with @YuHsuan-Hwang on the details and file a companion frontend issue?

[kswang1029]

fixed via #1868 and #1113.