[Discuss-gnuradio] Blank display in control port performance monitor.

Hi, I'm having problems with the Performance monitor, the graph output is blank.  The control port monitor is working fine.

The terminal output is the following:

ControlPort Monitor running.
ControlPort Monitor running.
gr::log :INFO: audio source - Audio sink arch: alsa
gr::log :INFO: controlport - Apache Thrift: -h Precision-7820-Tower -p 34612
aUaUaUmonitor::endpoints() = -h Precision-7820-Tower -p 34612
running: ['gr-ctrlport-monitor', 'Precision-7820-Tower', '34612']
aUaUmonitor::endpoints() = -h Precision-7820-Tower -p 34612
running: ['gr-perf-monitorx', 'Precision-7820-Tower', '34612']
  File "/usr/lib/python2.7/dist-packages/matplotlib/backends/backend_qt5.py", line 259, in mousePressEvent
    guiEvent=event)
  File "/usr/lib/python2.7/dist-packages/matplotlib/backend_bases.py", line 1903, in button_press_event
    self.callbacks.process(s, mouseevent)
  File "/usr/lib/python2.7/dist-packages/matplotlib/cbook.py", line 563, in process
    proxy(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/matplotlib/cbook.py", line 430, in __call__
    return mtd(*args, **kwargs)
  File "/home/grecia/rfnoc/bin/gr-perf-monitorx", line 837, in button_press
    i = nearby.index(min(nearby))
ValueError: min() arg is an empty sequence

The error only occurs when I click somewhere in the window of the performance monitor.