Figures, Axes & Line Plots

The problem, from first principles

Every matplotlib chart is a small tree of objects. A Figure is the canvas. It holds one or more Axes, each of which is one plotting area with its own x and y axis. Each Axes holds the things drawn on it: Line2D objects for lines, patches for bars, collections for scatter points, Text for labels. When you write plt.plot(...) you are using a shortcut that quietly creates a Figure and an Axes for you and draws into "the current one". That is fine for a notebook cell and a recipe for confusing bugs in a function that draws more than one chart.

This question uses the object interface, where you hold the Figure and the Axes yourself and call methods on them. Because a chart is a tree of objects with numbers inside, you can read those numbers back, which is also how it is tested: no pixels, just ax.lines[0].get_ydata().

From theory to code

Implement line_chart(x, y, title, xlabel, ylabel), add_line(ax, x, y, label, color), line_data(ax) and style_line(line, color, linestyle, linewidth, marker). The signatures and docstrings are already in the editor.

Constraints

• Create figures with plt.subplots() and draw through the returned Axes. Do not call plt.show().
• line_chart returns (fig, ax): a new Figure with exactly one Axes holding exactly one line through the points (x, y), with the given title, x label and y label, and the grid switched on.
• add_line draws one more line on ax through (x, y) with the given label and color, shows a legend listing the labelled lines, and returns the new Line2D.
• line_data(ax) returns a list with one (xs, ys) pair for every line on ax, in drawing order, where xs and ys are Python lists of numbers.
• style_line(line, color, linestyle, linewidth, marker) changes those four properties of an existing line (it does not draw a new one) and returns the line.

Hints