xlabel doesn't work in some versions of Python

**Describe the bug**
When using Python 2.7, the plot_chain_panel method encounters an error when using the xlabel.

**Expected behavior**
Ideally, the user can control the xlabel string and all settings (kwargs).

**Potential Solution**
Give the user the option to define an "xlabel" and "xlabelformat" setting.  This could then be correctly assigned when defining the xlabel within the code.