when zooming, set_axes_ylim only perform when total_points > 1 to prevent the...

sohstationviewer/view/plotting/plotting_widget/plotting_widget.py

@@ -586,12 +586,13 @@ class PlottingWidget(QtWidgets.QScrollArea):
                     if tr_min_ys != []:
                         new_min_y = min(tr_min_ys)
                         new_max_y = max(tr_max_ys)
+                        # in case total_points == 1, y lim shouldn't be set
+                        # again or the plot would be collapsed to one line
+                        if total_points > 1:
+                            self.plotting_axes.set_axes_ylim(
+                                ax, new_min_y, new_max_y)
                     ax.center_total_point_lbl.set_text(total_points)
 
-                if new_min_y is not None:
-                    self.plotting_axes.set_axes_ylim(
-                        ax, new_min_y, new_max_y, ax.chan_db_info)
-
     def draw(self):
         """
         Update drawing on the widget.