This graph shows the strength of Earth's magnetic field throughout during the past 800,000 years. Sometimes when the strength of the field falls below 4, Earth's magnetic poles switch places. The field has been weaker than that a few times in the past 800,000 years... but the magnetic poles haven't ever flipped during that time. The arrows show important "excursions", times when the magnetic field changed a lot. Geologists have given names to important excursions.
Image courtesy of the USGS.