Fix bug that acquires attribute chan on ax on the non plotting area

sohstationviewer/view/plotting/plotting_widget/plotting_widget.py

@@ -296,9 +296,9 @@ class PlottingWidget(QtWidgets.QScrollArea):
             focus SOH tab, roll to the corresponding line.
         """
         artist = event.artist
-        ax = artist.axes
-        chan_id = ax.chan
         if isinstance(artist, pl.Line2D):
+            ax = artist.axes
+            chan_id = ax.chan
             chan_data = self.plotting_data1[chan_id]
             # list of x values of the plot
             x_list = artist.get_xdata()