Display Graph of PVs alongside OPI

As a scientist, I want to display the history of a variable as a function of time on a graph alongside the GUI of a device, so that I can see what effect changing the device settings has on the variable.

#### Acceptance Criteria
1. I can display an empty graph alongside device GUIs (i.e. OPIs)
1. I can select one or more variables (blocks or PVs) to display on the graph.
1. The selected variables are shown as functions of time
1. I can save the graph as a file (e.g. as a PNG file)
1. I can copy/paste the graph to another application.

#### Notes
1. The aim is to be able put a device GUI next to a graph of one or more PVs. The PV need not be one directly associated with device itself (for example, conceivably, you might want to display the change in, say, scattered intensity as the voltage on the magnet power supply is varied.
1. For some scientists this is a "killer feature".
1. We could limit the number of variables the user wishes to display to, say, 5. Having more than 5 traces on the graph is likely to get confusing.
1. Attached is a very rough schematic of how the graph might appear.

No files or tests are named. Start by locating the existing OPI/device GUI entry point and the code that accesses PVs, then determine how a time-series graph can sit alongside the GUI. Done means selecting one or more variables, displaying their history over time, and supporting empty graphs, file saving, and copy/paste as described in the acceptance criteria.

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