Interpolating between data points

[21jackj]

Help.... perhaps there is an easy way, but I don't know of it.

I am trying to add two curves in a line chart. Example. I have one series with 3 data points with excel automatically connecting the 3 data points (January: 10 footballs sold, June: 20 footballs sold, November 40 footballs sold) I want a straight line between each of those 3 points. Excel does that. Series 2: (February: 50 baseballs sold, April 60 baseballs sold, December: 150 baseballs sold). so I plot the two curves, now I would like to add the curves and at each month displaying the total of the two.

I can think of two possibilities:

1) do it in the graph, does excel allow you to add two curves (remember the data I have are not in the same months, so I am looking to add the INTERPOLATED values together) - I don't think I have found a way to do that.

2) do it in the data table using a formula. As an example.

column A: months --- easy enough
column B: my few data points, with blank cells between the data points
column c: I need interpolated data for each month using a formula. that way I can plot this column and add this column to the other data column and then plot the resulting column. I don't know how to do this.

However, I want to be able to change the data points in column B - not only the values but the months which I have data. example.. I want to change the target of 50 baseballs in Feb to 50 baseballs in March, and I want column C to automatically update when I delete the 50 in the Feb cell and put it in the March cell, etc.

Another way of asking this question: is there a say of converting the line graph curve back to values at all X values and somehow getting it back into the data cells?

thanks for any help.