Orientation of y-axis in Heatmap

[hackermd]

The default orientation of the y-axis in the Heatmap graph is, at least to me, unexpected. The 0,0 coordinate of the plot is in the lower left corner. A heatmap is generally used to display a matrix, such as an image, however, and the 0,0 index into the matrix would be in the top left corner. An image is thus displayed flipped upside down. One can of course flip the z array or provide the values for y in reversed order, but this keeps the labelling of the axis and the display of y,x in the hover window unaffected. I like the way this is handled in matplotlib.pyplot.imshow, which provides an origin argument.
An additional argument for the Heatmap graph to control the orientation of the y-axis would be nice.

[hackermd]

Figured it out. Python example:

plotly.graph_objs.Layout(
    yaxis=dict(autorange='reversed')
)

[elena-pascal]

I agree, while one can do
yaxis=dict(
autorange='reversed'
)
this actually changes the handedness of the coordinate system you're working in. If the heatmap is not the final step, let's say you want to compare this against other data this easily becomes a nightmare. I would find it useful this flip of the origin to be made explicit.

[deliciouslytyped]

@elena-pascal did this ever get solved?

[elena-pascal]

@deliciouslytyped the issue had been closed a while back and I didn't reopen it just called into the void. Wasn't sure if it's affecting only me. Feel free to reopen if you feel strongly about it.

[deliciouslytyped]

I think I'd have to post a new issue.

[Shivamshaiv]

@elena-pascal I think something truly needs to be done, as I am in a point in a project when a lot of time got wasted just to find ways around.

[emmanuelle]

Hi @elena-pascal @Shivamshaiv @deliciouslytyped could you please explain why

yaxis=dict(
autorange='reversed'
)

is not a solution for you? Also note that px.imshow has an origin argument.

[empet]

The Plotly heatmap is intended to represent a discrete version of a scalar field, z= f(x, y), with x, y in a rectangle [a b] x [c, d].
All users participating in this discussion consider that a heatmap is generally used to display a matrix, such as an image, and pixel (0, 0) should be in the upper left corner.
This is the case when using matplotlib. Matplotlib defines a heatmap as an image, but its imshow provides the possibility to set the origin as 'lower' or 'upper'. With origin='lower', you'll get the equivalent of the default plotly heatmap, while origin='upper' has the same effect as setting yaxis_autorange='reversed'

Generally people consider as being the "default definition" that given in a library they used before.

[elena-pascal]

Hi @elena-pascal @Shivamshaiv @deliciouslytyped could you please explain why

yaxis=dict(
autorange='reversed'
)

is not a solution for you? Also note that px.imshow has an origin argument.

It is the matrix representation aspect as described above. Simply reversing the axis messes up the handedness (I need to differentiate between front and back too) of the plotted matrix, and for a complicated geometry loosing this bit of sanity is undesirable.

[emmanuelle]

Thank you for your answer, but I don't understand the "handedness". Maybe a schematic drawing would help? Did you try passing your data to px.imshow? It should behave exactly like matplotlib's imshow with the same default for the origin.

[elena-pascal]

Most matrix maths implies right handed coordinate Cartesian systems (the left image). Flipping one axis is such a system results in having a left handed coordinate system (the right image). Moving from one to another is a pain as the definition of operations, eg rotation, must change. I know this can be a problem because we tried for years to compare different handedness images and left many scratching their heads.

Tbh, I don't quite remember now why I wanted to use heatmap and not imshow.

[deliciouslytyped]

Is there some reason this would be hard to implement? I don't remember the details - it's been a long time since I fought with this issue - but it should just be a simple coordinate transformation on the backend that if we try to fix from the calling side, ends up complicating things unnecessarily.

Since there's things like

plotly.graph_objs.Layout(
    yaxis=dict(autorange='reversed')
)

I see no reason why what we want shouldn't be.

TL;DR: I don't remember what the problem was, but I strongly believe it's valid and I don't see why it would be difficult to fix. And It should be fixed in the library. IIRC it's a major usability pain point if you need it.

[deliciouslytyped]

I'll just open a new issue pointing to this one.