Create Contour Plot from Pandas Groupby Dataframe

Question

I have following Pandas Dataframe:

In [66]: hdf.size()
Out[66]:
a           b
0           0.0          21004
            0.1         119903
            0.2               


        

Answer 1

Welcome to SO.

It looks quite clear that for each of your 'a' level, the numbers of 'b' levels are not the same, thus I will suggest the following solution:

In [44]:

print df #an example, you can get your dataframe in to this by rest_index()
    a  b     value
0   0  1  0.336885
1   0  2  0.276750
2   0  3  0.796488
3   1  1  0.156050
4   1  2  0.401942
5   1  3  0.252651
6   2  1  0.861911
7   2  2  0.914803
8   2  3  0.869331
9   3  1  0.284757
10  3  2  0.488330

[11 rows x 3 columns]
In [45]:
#notice that you will have some 'NAN' values
df=df.pivot('a', 'b', 'value')
In [46]:

X=df.columns.values
Y=df.index.values
Z=df.values
x,y=np.meshgrid(X, Y)
plt.contourf(x, y, Z) #the NAN will be plotted as white spaces
Out[46]:
<matplotlib.contour.QuadContourSet instance at 0x1081385a8>

Answer 2

Thanks a lot! My fault was, that I did not realize, that I have to apply some function to the groupby dataframe, like .size(), to work with it...

hdf = aggdf.groupby(['a','b']).size()
hdf

gives me

a           b
1           -2.0          1
            -1.9          1
            -1.8          1
            -1.7          2
            -1.6          5
            -1.5         10
            -1.4          9
            -1.3         21
            -1.2         34
            -1.1         67
            -1.0         65
            -0.9         94
            -0.8        180
            -0.7        242
            -0.6        239
...
187          0.4        22
             0.5        10
188         -0.6         2
            -0.5         2
            -0.4         1
            -0.3         2
            -0.2         5
            -0.1        10
            -0.0        18
             0.1        19
             0.2        20
             0.3        13
             0.4         7
             0.5         5
             0.6         1
Length: 8844, dtype: int64

With that, and your help CT Zhu, I could then do

hdfreset = hdf.reset_index()
hdfreset.columns = ['a', 'b', 'occurrence']
hdfpivot=hdfreset.pivot('a', 'b')

and this finally gave me the correct values to

X=hdfpivot.columns.levels[1].values
Y=hdfpivot.index.values
Z=hdfpivot.values
Xi,Yi = np.meshgrid(X, Y)
plt.contourf(Yi, Xi, Z, alpha=0.7, cmap=plt.cm.jet);

which leads to this beautiful contourf: