BUG: Seed RNG in test_partially_invalid_plot_data

Came up upon merging another PR. See comment https://github.com/pydata/pandas/pull/6644#issuecomment-38057195

The test relies on drawing at least 1 draw out of 10 uniform draw to be larger than 0.5; even if this didn't cause the failure originally, it should still be fixed. I set a seed that's known to produce the desired values.