Line of best fit for two sets of data.

Hi there. I really need some help. i have a lab report due for Uni and ive got the graph plotted and generally up and running, theres just a few things i need to tweek. i have two different sets of data plotted and need to get the line of best fit up. i know how to get a line of best fit for each seperate data set (so two lines of best fit in total) but not one between them.
Also...how would i get a micro sign on the axis titles? it wont let me add in a symbol.
i would greatly appreciate some help.
hope to hear from someone soon

you have to change your data so it all appears to be from one source, and then do the line of best fit - but statistically, it sounds very dubious to me, you will be in effect hiding information....