Allow for ".vary_params" in Visualizer library functions

[tiffanymtang]

Allow use of ".vary_params" or another keyword to refers to the varying parameter in plot_eval_constructor(). For example, if we are varying over "n" in the experiment and we want to plot "n" on the x-axis, we might want to say plot_eval_constructor(x_str = ".vary_params", ...) instead of plot_eval_constructor(x_str = "n", ...). Might also want to extend this for multiple varying parameters.