Plot probability along a time line for 3 states

Does any one have an example for showing the probability of 3 states over time. So a random variable has 3 possible states each with some probability, and the total probability is of course 1. I want to show a 3 line graphs with the top line always at value 1. The space between lines show the prob of a given state at any time t. I like different color for background of each state.

I think what it involves is drawing 3 lines (curves) but fill the area under the line to be colored. Then allow the lower state curve to be at a higher plane, covering the background of the higher state curves. Please give some advice what’s the api for doing this.