Start with the axes
Every line chart has a horizontal axis (usually time, left to right) and a vertical axis (the value the line shows). Read the axis labels before judging the line. The same wiggle can look dramatic on a zoomed-in scale and flat on a zoomed-out one, so the vertical range sets the context for everything else.
What the slope tells you
The steepness of the line is its rate of change. A line climbing to the upper right means the value is rising; a line dropping to the lower right means it is falling. A flat line means the value is steady. Where the line changes from rising to falling (or the reverse), the trend has turned — those turning points are usually the most interesting moments in the data.
Comparing two lines
When a chart shows two lines, the gap between them matters more than either line alone. If your drawn guess line sits close to the revealed actual line, you read the trend well; a wide gap means the real outcome surprised you. In Extraplot that distance is exactly what your score measures.
Watch for scale tricks
Truncated axes (where the vertical axis does not start at zero) can exaggerate changes, and uneven time spacing can hide or invent patterns. Before you guess what comes next, confirm the axis starts, ends, and steps are labelled honestly. A calm read of the scale beats a confident guess every time.