Add a `sort` and `aggregate` option to line layers

A line layer should have `sort` and `aggregate` options.

If there are multiple `y` values for a particular `x` value, the `aggregate` option defines how to plot those `y` values as a single point. It should support common aggregate operations like `mean`, `median`, `mode`, `max`, `min`, etc.

`sort` defines the ordering of points within a group in the table (which controls the ordering of line segments in the plot).

Start by locating the line-layer implementation and the code that groups table rows by x values. Trace how points are ordered and how multiple y values are currently plotted. Done means line layers accept sort and aggregate options, support common aggregation operations such as mean, median, mode, max, and min, and produce the specified point ordering.

Written by the indexing model from the issue text.