Excel - setting minimum value for smooth curve

How do I prevent smooth curve in Excel from going below 0 on a graph? I need a smooth curve because I don't have enough data points, it looks terrible as a straight line. As you can see on this graph two of the data series goes below 0, even though none of the data points are 0.

graph

I don't want to split it into 2 data series, that wouldn't make any sense as it would show up as 2 items in the legend.