I'm not sure that we're on the same page - it's not a straight line /
curved line problem, it's a "style" of interpolation.

For example, if I had the following data:-

x y
0 5
1 6
3 7

If I wanted the x value for x=0.5, y should be 5 (i.e. y = 5 for x = 0
to 1), not 5.5 (if it was interpolated)

I know I can do this, if I fiddled with the data and made a series
like so:-
x y
0 5
0.99 5
1 6
2.99 6
3 7

But that seems a lot of work for something that I want to do
repeatedly.... I would have thought that Excel might have another
option for drawing lines between the points. The chart type is closer
to a column chart, but the x values are not evenly spaced, so I can't
use that.

Thanks,
Kate