I have run a linear regression association test on PLINK, and then I have represented the results with a Manhattan plot. The problem is that a strange horizontal alignment appears in the representation (around Pvalue = 10^(-7)). I tried to run a second association test with another phenotype, however this horizontal "line" still persists (this time at the top of the graphic). It seems to be an artefact, but I can't identify the cause. What could be the problem?