Re: [Matplotlib-users] problem with design of matplotlib.pyplot.scatter?

[Phillip M. F. <pfe...@ve...>]

Hello Eric-

I've looked at the code in colors.py.  I think that I'm starting to 
understand what's going on, but I'm unclear about a few things.  In 
particular:

- Why do we need to define both forward and reverse transformations?  
Shouldn't the forward transformation be sufficient?

- I don't follow what the snippet of code below is doing:

        if cbook.iterable(value):
            vtype = 'array'
            val = ma.asarray(value).astype(np.float)
        else:
            vtype = 'scalar'
            val = ma.array([value]).astype(np.float)

- In some cases I'd like to map the data values to discrete output 
values, e.g., values below x_0 map to 0 (which the colormap in turn maps 
to red), values between x_0 and x_1 map to 0.5 (which maps to yellow), 
and values greater than x_1 map to 1 (which maps to green).  Such a 
function does not have a mathematical inverse because it is a many to 
one mapping.  How does one handle this situation?

The ability to pass in an ordinary function (or a pair of functions if 
the inverse is really necessary) would be a great benefit.

Thanks!

Phillip

Eric Firing wrote:
> Dr. Phillip M. Feldman wrote:
>> I'd like to generate a scatter plot in which symbols are colored using a
>> specified colormap, with a specified mapping from the range of the 
>> data to
>> the [0,1] colormap interval. I thought at first that one could use 
>> the norm
>> argument to specify a function that would perform this mapping, but from
>> closer reading of the documentation (and experimentation) it seems as 
>> though
>> one cannot do this. Is there another mechanism for doing this? (I could
>> remap the data itself before plotting it, but this is unacceptable 
>> because
>> the colorbar tic lables would then take values in [0,1] rather than 
>> values
>> from the range of the data).
>>
>
> I don't understand--what you say you want to do is exactly what the 
> norm is designed for.  Maybe the problem is that you can't pass in a 
> simple function--you need to subclass colors.Normalize.  An example is 
> colors.LogNorm.
>
> It looks like what we need is a FuncNorm, which would be initialized 
> with two functions, the forward and inverse transformation functions, 
> each taking vmin, vmax, and a val.
>
> Eric
>