Colormap norming

Color mapping and scalar bar feature requests:

These requests stemmed from conversations with @prisae and @craigmillernz

Custom Colormaps

Can we implement a way for a user to make custom colormaps using matplotlib and send that map as the cmap argument? This should be as simple as adding in a type check and skipping over the colormap lookup code.

Update: implemented in #126

Categorical Colormaps

We also need a way to have categorical colormaps

Update: this is possible by using a custom built colormap from matplotlib

norm option

Also, could we add a new norm keyword argument similar to matplotlib? See this page. This would help users apply a standard matplotlib norm or center a color mapping around a value without changing the range. E.g “My data scale goes from -0.6 to 0.3 and I’d like the red/blue color map to be centered so that white is at 0.”

@craigmillernz provided a super useful function for implementing this type of norm:

#Class to normalize colors to center around a value
from matplotlib.colors import Normalize

class MidpointNormalize(Normalize):
    def __init__(self, vmin=None, vmax=None, midpoint=None, clip=False):
        self.midpoint = midpoint
        Normalize.__init__(self, vmin, vmax, clip)

    def __call__(self, value, clip=None):
        # I'm ignoring masked values and all kinds of edge cases to make a
        # simple example...
        x, y = [self.vmin, self.midpoint, self.vmax], [0, 0.5, 1]
        return np.ma.masked_array(np.interp(value, x, y))

Interactive colormaps

Could we maybe make a tool to create custom colormaps? This honestly might make the most sense to create a totally separate module under @OpenGeoVis. I’m thinking some sort of ipywidget or PyQt5 type interface where a user can harness to create a custom colormap. If we can get this going, then provide a way for it to like with vtki and update the mappers so a user can interactively update the rendering scene while changing the colormap.

Here is an example from @prisae in MatLab:

[prisae]

LogNorm and SymLogNorm

Another point (along comments by @craigmillernz): Adding the possibility for LogNorm and SymLogNorm (as in matplotlib/discretize).

[prisae]

Thanks for opening the issue. As I mentioned on Slack, the idea arose because you mentioned the interactive keyword, which I meant was about changing the colormap, but turned out to be about changing the appearance/location of the colorbar. Again as I said before, I think in general this is a bad idea (at least for static figures, for the exactly same reasons why jet etc are bad colormaps), but it can be very useful to explore the dataspace interactively.