include an additional argument in `graph.curve.params()` to control the color of the chains.

[d-morrison]

Currently, each chain is plotted with a different color, but we would like to make this behavior optional — so users can choose between showing different colors for each chain or using the same color (e.g., black) for all chains regardless of how many there are.

Originally posted by @Kwan-Jenny in #434 (comment)

[Kwan-Jenny]

I think there should be an argument that allows users to control the x-axis range. Therefore, in the next PR branch, I plan to add the following three arguments:

1. Option to display each chain in black or in different colors, based on user preference.
2. Option to include or omit the median curve for each chain.
3. An argument to control the x-axis range.

I will ask to check with @d-morrison later when Dr.Morrison has time.