MAVExplorer: Unable to graph instanced MAVLink messages

When looking at tlogs instanced messages fail to graph, they just open a blank window with no error messages. For example on a tlog with 4 instances of BATTERY_STATUS (all of which can be seen in the dumped log) `MAV> graph BATTERY_STATUS[0].current_battery` opens an empty window.

Start by reproducing the reported command, `MAV> graph BATTERY_STATUS[0].current_battery`, against a tlog containing multiple BATTERY_STATUS instances. Trace the MAVExplorer graph handling for instanced messages and compare it with the dumped log output. Done means the command opens a populated graph rather than a blank window.

Written by the indexing model from the issue text.