correct time stamps of plot tickers

time stamps were counted with fps=1, fps=4 is currently used for the extraction
of frames with ffmpeg

itten/visuals.py

@@ -21,7 +21,18 @@ class SequencePlot(object):
                                    view._frame_step)]  # um X-Achse Minuten anzeigen zu lassen
 
     def _timelabels(self, val, pos):
-        min, sec = divmod(int(val), 60)
+        """counts time values for plot tickers
+        
+        Arguments:
+            val {int} -- default ticker value (frame number) as used by matplotlib
+            pos {int} -- current ticker position as passed by the matplotlib.FuncFormatter
+        
+        Returns:
+            str -- timecode for the given ticker label (frame number)
+        """
+
+        val, _ = divmod(int(val), 4)  # scale frame number from fps=4 to fps=1
+        min, sec = divmod(int(val), 60)  # calculate timecode
         timelabel = "{0}:{1:02d}".format(min, sec)
         return timelabel