issues selecting some attributes

Experienced this error in Positioner plots, but might show up in others. A couple issues:

1. Sometimes when select a new attribute an Re-Plot, nothing happens
2. With some attributes, get this error:
`error: ValueError('On entry to DLASCL parameter number 4 had an illegal value')`

Start with the Positioner plot attribute-selection entry point and reproduce both reported behaviors by selecting different attributes and clicking Re-Plot. Trace the resulting plot update and the path that raises the DLASCL ValueError. Done means every supported attribute selection updates the plot without the reported error.

Written by the indexing model from the issue text.