ENGIN-X / IMAT: Stress rig: Displaying channel histories

As a scientist, I want to display X-Y graphs of selected channel data (stress, strain and position), so that I can see how they vary w.r.t. each other.

#### Acceptance Criteria
1. I can select stress, strain or position to be the X-axis of the graph.
1. I can select stress, strain or position to be the Y-axis of the graph.
1. The system plots Y against X and updates the graph periodically.
1. I can clear the graph in order to start accumulating data for a new graph.
1. The graph should be persisted between restarts of the OPI, IOC or IBEX client.

#### Notes
1. See also #2438 and #2439
1. Obviously, if the user selects one variable for the X-axis, that variable cannot then be selected for the Y-axis (otherwise the result would be a straight line).
1. The refresh period may need to be determined by trial & error. Too rapid a refresh rate will impact performance; too slow might be unsatisfactory for the scientists.
1. In order to persist the data between re-starts, the accumulated data will need to be cached
1. The ability to clear the graph should be provided by a "Clear Graph" button.
1. On pressing the "Clear Graphs" button, the cache can be cleared
1. On pressing the "Clear Graphs" button, the system should continue to accumulate data and to graph the new data.
1. As the PV history grows, the number of points will grow. If the number of points grows too large we may need to consider pruning the data.