Making a trend line for several series of data
 

Well, for my chem and phsyics classes I will probably be making a lot of
graphs, so I'd rather do them on the computer. The problem is, when I use
excel, there is no way I can find a trend line for ALL the data series I
have. I have to pick one line and find the trend line for that. Is there any
way I can do that??
Also, can I make an error bar that goes through, for example, 3 data points
that are in the same place on the x-axis? Because from what I can tell error
bars have to be the same for all of the points, and cannot be placed
separately. So far I have been doing both these things by placing regular
lines on the graph, but this isn't exactly accurate!
Please help me! :S