Linear interpolation

I have some data from a test I need to find the point that the subject crosses, first inflection, 2.0, 3.0, 4.0 and last mark above 4.0. (Column D) on the test then give the interpolated value from (column A and C)
A C D
Means Sum Increase Point
100 130 0.9
130 133 1.1
160 142 1.2
190 153 2.4
220 160 4.3
250 168 7.2
280 173 9.1


I then need to plot this in the graph at the 2.0 mark and give the number the vertical line crosses from the interpolation in a chart below the graph. I need to do the same thing at first inflection, 2.0, 3.0, 4.0 and the point just above 4.0 or in this case 4.4. Then plot vertical lines at each point on the graph. I attached a sample excel sheet for reference, it does not have any macros or other bugs.

As I have the numbers already for this sample they would look something like this:
First Inflection (i.e. Above 1.0 and significant increase in column C) This one is easy as it is always the exact values.
Means=160
Sum=142

2.0
Means 182
Sum 150

3.0
Means 200
Sum 156

4.0
Means 216
Sum 159

Above in this case 4.3
Means 222
Sum 161

Thanks for any help you can give.