Add legend to pgf plot results in \end{axis} error

Expanding my comment to an answer.

The problem is in the use of forget plot. Due to the fact pgfplots shall forget each plot, but on the other hand defining a legend entry for each plot, pgfplots doesn't know what to do. To solve the problem two possibilities are given, based on the desired result.

1. Remove all \addlegendentry from the different plots, so no legend will be plotted.

2. Remove -at least one- forget plot to plot the legend.