Graphing in Excel

I have never been able to convince Excel to draw the graphs that I expect it should draw. I have a set of data which I want to use for the values of x, and another set that I want to use for y, and a third set to graph which shows the relation of x to y. Is this too much to ask?

The particular application in this case is to show how the cost of a house changes in relation to changes in the rate of taxation and in the financed price of the property, with the mortgage rate being a fixed value. The curve should be relatively flat; that is, as the price goes down, and the taxes go up, the costs remain similar.

So what am I missing?