Show time label on top of ruler

Closes #71 (closed)

sohstationviewer/view/plotting/plotting_widget/plotting_widget.py

@@ -379,8 +379,10 @@ class PlottingWidget(QtWidgets.QScrollArea):
             pass
         self.set_ruler_visibled(self.ruler, xdata)
         if xdata >= self.min_x:
+            ruler_text_content = format_time(xdata, self.parent.date_format,
+                                             'HH:MM:SS')
             self.ruler_text = self.plotting_axes.fig.text(
-                xdata, 75, 'Potato',
+                xdata, 75, ruler_text_content,
                 verticalalignment='top',
                 horizontalalignment='center',
                 transform=self.timestamp_bar_top.transData,