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A little funkyness with Pandas index

Problem:

I made a Pandas DataFrame 
df
with a float index. Everything worked fine, including 
df.plot()
But when I tried to do 
pylab.exp(df.index.values)
I got the mysterious error 
AttributeError: exp

Explanation:

It turns out that Pandas' index is restricted to int64 and 'object' and so 
df.index.values
returns an 
'object'
type array with exp can't interpret. We need to explicitly cast it as 
'float'
to get things to work : 
pylab.exp(df.index.values.astype('float'))
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