Viewing the graph of a full data file or adding more sensors to the view of a graph (through the sensor selector) may occasionally draw a blank graph. Try doing a small pinch to refresh the plot and correctly draw all series. If this does not work, try hiding sensors (tap the menu icon at the top of the graph) to see if the plot redraws correctly. You can also tap Done and then tap the mini-graph again to see if this corrects the issue. If not, restart the app and try viewing the graph again.