Time Series Graphs

Step-by-Step Construction

• Axis Setup: Label the x-axis with the time units and the y-axis with the quantity being measured. Ensure the scales are linear and cover the full range of data.
• Plotting: Mark each data point as a small cross or dot at the intersection of the time and the recorded value.
• Connecting: Use a ruler to join the points in chronological order from left to right.

Choosing Line Types

• Solid Lines: Used when the data represents a continuous process where values exist at every moment between the points (e.g., temperature).
• Dashed Lines: Used when the values between points are unknown or when the intermediate points are meaningless (e.g., total sales at the end of each day).

It is important to distinguish time series graphs from other statistical visualizations to ensure the correct analysis is applied.

• Time Series vs. Scatter Graph: While both plot points, a time series graph always connects points in a specific order to show a sequence, whereas a scatter graph looks for correlations without necessarily implying a temporal sequence.

• Check the Scale: Always verify that the time intervals on the x-axis are equal. If the data skips a month or year, the gap on the graph must reflect that missing time to avoid distorting the trend.

• Labeling Precision: Ensure both axes have clear titles and units (e.g., 'Time (Months)' and 'Revenue (USD)'). Missing units often result in lost marks.

• Trend Identification: Look for the 'big picture'. If the points generally move upwards, it is a rising trend; downwards is a falling trend. Repeating 'peaks and troughs' suggest seasonal or cyclical patterns.

• Reasonableness Check: If the graph shows a sudden, massive spike or drop, double-check the plotted coordinates against the data table for transcription errors.

• Irregular Spacing: A common mistake is spacing points equally on the x-axis even if the time intervals in the data are irregular (e.g., plotting Jan, Feb, and then June with the same distance as Jan to Feb).

• Incorrect Line Choice: Students often use solid lines for discrete data where the 'in-between' values don't exist. Using dashed lines is more accurate for data like 'monthly club membership totals'.

• Starting at Zero: While the y-axis often starts at zero, it is not always mandatory if the data values are very high. However, failing to use a 'break' symbol in the axis can make small changes look deceptively large.