MRTG MaxBytes Setting Can Cause Graphs to Flat Line

We use MRTG on our internal network to help monitor bandwidth for several network connections.  After setting up monitoring for several VPN’s I noticed a couple of the busier VPN’s would “flat line” during the day but look normal after hours.  When MRTG can’t access the device to get data, it continues to plot the previous value which creates this flat line effect.  Therefore I initially thought there was a problem with those VPN’s, but the VPN’s were working fine.  After digging further I noticed the MRTG cfgmaker utility put  “MaxBytes[xxxx]: 1125” for the graph of the VPN interface.  This tells MRTG how to scale the graph, but it also discards data above that threshold.  So during the day the bandwidth was above the threshold and displaying the flat line graph because it was displaying the last known good data value.  I increased the value on the MaxBytes statement to fix the problem.