Annotations add text or markers to the chart to draw attention to a specific point: ax.annotate places text with an arrow, ax.axhline and ax.axvline draw reference lines, ax.text places a label. They turn a chart into a message, as long as they stay restrained.
Example
ax.annotate('Peak', xy=(date, valeur), xytext=(date, valeur + 5), arrowprops=dict(arrowstyle='->'))