pandas groupby boxplot red diagonal line

I'm plotting a box plot with

>>> score.head()
       Score  Wafer_Slot
0 -10.710504          14
1   1.444185          23
2  -5.727797          14
3   8.029544          21
4  -1.569401          24

gscore = score.groupby('Wafer_Slot')
xticks = []
for i in range(25):
    xticks.append(str(i+1))
    xticks.append(' ')

gscore.boxplot(subplots=False)
ax = plt.gca()
ax.set_xticklabels(xticks)
plt.ylabel('Anomaly Score', size=20)
plt.xlabel('Wafer Slot', size=20)
plt.show()

The plot is fine but there is a dashed red diagonal line. What is this line? How can I remove it?

Obtained figure: