#!/usr/bin/env python3
"""Plot instrument. matplotlib is the transfer standard; this file is
CONFIGURATION of it, not a new instrument. It adds exactly one policy:

    the y-axis includes zero unless truncation is explicitly requested,

because an autoscaled y-axis turns a correct dataset into a false picture, and
that is the most common way a plot lies.
"""
import sys, csv, matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
import matplotlib.dates as mdates
from datetime import datetime

def plot(series, out, title, ylabel, truncate_y=False, dates=False):
    fig, ax = plt.subplots(figsize=(11, 5.2), dpi=160)
    for name, xs, ys, color in series:
        ax.plot(xs, ys, marker="o", ms=2.6, lw=1.4, color=color, label=name, zorder=3)
    if not truncate_y:
        ax.set_ylim(bottom=0)                      # the policy
    if dates:
        ax.xaxis.set_major_locator(mdates.AutoDateLocator())
        ax.xaxis.set_major_formatter(mdates.DateFormatter("%b %d"))
    ax.set_ylabel(ylabel); ax.set_title(title, loc="left")
    ax.grid(True, lw=0.5, alpha=0.35, zorder=0)
    ax.legend(frameon=False, loc="upper left")
    for s in ("top","right"): ax.spines[s].set_visible(False)
    fig.tight_layout(); fig.savefig(out)
    return ax

def readings(ax):
    """What the plot ACTUALLY contains — read back off the axes, not off the
    input. This is what makes the plot checkable."""
    out = []
    for ln in ax.get_lines():
        d = ln.get_xydata()
        out.append({"label": ln.get_label(), "n": len(d),
                    "xmin": float(d[:,0].min()), "xmax": float(d[:,0].max()),
                    "ymin": float(d[:,1].min()), "ymax": float(d[:,1].max()),
                    "last": (float(d[-1,0]), float(d[-1,1]))})
    return out, ax.get_ylim()