[query/ggplot] add support for MatrixTables

[iris-garden]

Currently, ggplot expects a Table to be passed in as its data, and errors if passed a MatrixTable. With this change, the following code:

import hail as hl
mt = hl.utils.range_matrix_table(10, 10)
mt = mt.annotate_entries(entry_idx = mt.row_idx + mt.col_idx)
mt.show()

Which produces a MatrixTable that looks like this:

+---------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+
| row_idx | 0.entry_idx | 1.entry_idx | 2.entry_idx | 3.entry_idx | 4.entry_idx | 5.entry_idx | 6.entry_idx | 7.entry_idx |
+---------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+
|   int32 |       int32 |       int32 |       int32 |       int32 |       int32 |       int32 |       int32 |       int32 |
+---------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+
|       0 |           0 |           1 |           2 |           3 |           4 |           5 |           6 |           7 |
|       1 |           1 |           2 |           3 |           4 |           5 |           6 |           7 |           8 |
|       2 |           2 |           3 |           4 |           5 |           6 |           7 |           8 |           9 |
|       3 |           3 |           4 |           5 |           6 |           7 |           8 |           9 |          10 |
|       4 |           4 |           5 |           6 |           7 |           8 |           9 |          10 |          11 |
|       5 |           5 |           6 |           7 |           8 |           9 |          10 |          11 |          12 |
|       6 |           6 |           7 |           8 |           9 |          10 |          11 |          12 |          13 |
|       7 |           7 |           8 |           9 |          10 |          11 |          12 |          13 |          14 |
|       8 |           8 |           9 |          10 |          11 |          12 |          13 |          14 |          15 |
|       9 |           9 |          10 |          11 |          12 |          13 |          14 |          15 |          16 |
+---------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+
showing the first 8 of 10 columns

Can be plugged into ggplot like so:

from hail.ggplot import *
fig = ggplot(mt, aes(x=mt.row_idx, y=mt.entry_idx)) + geom_point()
fig.show()

To produce this plot:

This is accomplished using Expr._to_table to transform the MatrixTable such that the fields used to generate the plot can be straightforwardly selected from it.

[tpoterba]

awesome! a couple test comments

[tpoterba]

could we add a test that we get the right output for non-entry-indexed fields? mt.row_idx and mt.row_idx * 2 for instance (I want to make sure we get 3 points, not 9)?

Also, a sneaky way this testing pattern can be violated is if the lengths of expected isn't the same as data.x or data.y. Let's add an assertion that the lengths of all three agree.