Chart data incorrectly shown
 

I have three sets of data, two of which are being plotted just fine on the xy
scatter graph. The one set, however, won't start at my x value of 0. The y
values are working fine. X values go up in increments of 0, 0.5, 1, 1.5 etc
all the way up to 12 minutes, but with no 11.5 minute data, so rows 1-24.
Between the six and six and a half minute mark, the graph goes hay-wire and
shifts away from it's zero start point. I check x1, y1 on my graph and it is
actually using x3 with y1. If this makes any sense to anybody please help.
thanks.