bins and histograms

I'm having some troubles understanding basic concepts.
Suppose I want to do something like this, given a dictionary

values = { (0,10) : 0.5,
          (10, 20) : 0.3 }
and so on, where the key is a time slot interval and the value is the
value I want to plot.

What should be the correct way to get what I want?

In theory the data that I want to plot is [0.5, 0.3]
but if I do


I don't get that, but I get 2 bars of length 1 located in that x
coordinate. So how should I proceed then?