Plot values from startup

[chapulina]

Desired behavior

The plotting GUI plugin (https://github.com/ignitionrobotics/ign-gui/issues/66) is very powerful and can plot values from Ignition Transport as well as components. When using it repeatedly to check the behaviour of the simulation, it would be very convenient to configure it to plot specific variables from startup. This would save the manual work of clicking through the plugin and dragging values in every time.

Alternatives considered

Keep clicking through the GUI to plot the values of interest

Implementation suggestion

The configuration would come through the <plugin> tag, for example:

      <plugin filename="Plotting" name="Plotting">

        <transport
          topic="/topic" 
          field="field:nested_field"/>
        <component 
           entity="scoped_name" 
           type="ign_gazebo_components.Pose" 
           field="position:x" />

      </plugin>