Trend, season, and noise
Three things shape any time series. The trend is the slow overall direction. Seasonality is a repeating pattern (like warmer summers). Noise is the random day-to-day jitter. When you guess the missing part of a chart, you are really predicting whether the trend continues, the seasonal pattern repeats, or both.
Look at the longest stretch you can
A single dip or spike is often noise. Step back and read the whole visible line: is it generally rising, falling, or flat? The most reliable guess usually extends the dominant pattern rather than chasing the last wiggle.
Match the shape you were shown
If the visible part repeats a regular wave, expect the next cycle to resemble the last one. If it climbs steadily, a gentle continuation is a safer bet than a sudden reversal. Anchoring your guess to the shape already on screen beats inventing a new story.