La méthode savefig génère des graphes étranges

from math import sqrt, ceil
import matplotlib.pyplot as plt

class Drawer:
    def __init__(self, title=""):
        self.title = title
        self.clear()

        self.X_INDEX = 0
        self.Y_INDEX = 1
        self.TITLE_INDEX = 2
        self.XLABEL_INDEX = 3
        self.YLABEL_INDEX = 4

    def add(self, x, y, title="", xlabel="x", ylabel="y"):
        self.data.append((x, y, title, xlabel, ylabel))

    def clear(self):
        plt.clf()
        self.data = []

    def draw(self):
        l = len(self.data)
        cols = int(ceil(sqrt(l)))
        lines = int(ceil(l/cols))

        fig = plt.figure(num=1, figsize=(4.5, 3))
        fig.suptitle(self.title)

        for k in range(l):
            x = self.data[k][self.X_INDEX]
            y = self.data[k][self.Y_INDEX]
            title = self.data[k][self.TITLE_INDEX]
            xlabel = self.data[k][self.XLABEL_INDEX]
            ylabel = self.data[k][self.YLABEL_INDEX]

            plt.subplot(lines, cols, k+1)
            plt.plot(x, y)
            plt.xlabel(xlabel)
            plt.ylabel(ylabel)
            plt.grid(True)
            plt.title(title)

        plt.tight_layout() # To space subplots
        plt.subplots_adjust(top=0.9) # Not to overwrite title

    def save(self, dest, clear=True):
        plt.clf()
        self.draw()

        plt.savefig(dest, dpi=250)

        if clear: self.clear()

    def show(self, clear=True):
        plt.clf()
        self.draw()

        plt.show()

        if clear: self.clear()


if __name__ == "__main__":
    drawer = Drawer("TITLE")

    n = 6
    for i in range(n):
        drawer.add(list(range(i+2)), list(range(i+2)), str(i), "x" + str(i), "y" + str(i))
    drawer.show()

    import sqlite3
    from selector import Selector
    from config import DATABASE_PATH
    from workers import Productions

    selector = Selector(sqlite3.connect(DATABASE_PATH))

    for cow in [3129, 14, 3138, 1]:
        drawer = Drawer(cow)
        worker = Productions(selector, selector.get_lact_days)
        data = worker.work(cow)

        for line in data:
            x = line["x"]
            y = line["y"]
            title = "Lact: {}".format(line["lact"])
            drawer.add(x, y, title, xlabel="Day", ylabel="Production (L)")

        drawer.save("./{}.by-day.png".format(cow))

plt.tight_layout() # To space subplots
plt.subplots_adjust(top=0.9) # Not to overwrite title