The answer to all four questions is yes.
You can execute several plots in the same figure, as well as on the same axes (axes is what matlab calls what you would call a graph ;). If you have your date data in X, and your glucose readins in Y1, Y2 and Y3, say, you can either;
In regards to dates on the x axis, type doc datetick in the command window to open the documentation for datetick.
Basic functions like xlsread can read data from Excel sheets, and there are plenty of builtin functions to add trend lines to plots.