Rendering the traces of interest

Describe the features

1. when visualizing traces, I noticed that for my tensorflow examples, the only GPU trace line (the last trace line) isn't visible in the home view. I suggest that we use a new approach when visualizing a rectangle containing more than one trace line to make sure that the first and last trace lines in the rectang are always visible. This is particularly important for the home view. This will mean a slight adjustment to the scale factor being used in the rendering.
2. when filtering traces, I would like to be able to filter all traces that have an inconsequential size. for tensorflow, I have found many threads with essentially an empty trace. Ideally, I would like to be able to graph the number of entries in each trace and then select a cutoff after seeing the graph.