Plots.jl: how to plot a function with different values for a parameter

Hello, thank you so much for your solutions that I also could follow as I am a newbie here, but just some naive questions:

(1) how could you find out really nice different values for parameter a with a given domain and range of variable x in your figure? (do we need to run the regression to find out this?)

(2) in case of multiple parameters, how could we deal with this?
Thank you in advance!!