warn in history plot if state data is all nan

locklost/plots.py

 import io
 import sys
+import logging
+import numpy as np
 import matplotlib.pyplot as plt
 import matplotlib.dates as dates
 import xml.etree.ElementTree as ET
@@ -71,6 +73,8 @@ def plot_history(path, lookback='7 days ago', draw_segs=False):
         channel += '.mean,s-trend'
     bufs = data.nds_fetch([channel], gtmin(start), gtmin(end))
     state_index, state_time = bufs[0].yt()
+    if np.all(np.isnan(state_index)):
+        logging.warning("state data [{}] is all nan??".format(channel))
 
     # find events
     events = list(find_events(after=start.gps()))