Line Graphs and Change
Reading a line graph
A line graph shows how a quantity changes over time. Time runs along the horizontal axis and the measured quantity up the vertical axis, and the points for each moment are joined by a line. To read a value, find the time on the horizontal axis, go up to the line, and read across to the vertical axis. A temperature graph might show the temperature at six in the morning, at nine, at noon and so on, with the line joining them. Reading single values is the first step in making sense of a line graph.
Rising, falling and steady
The shape of the line tells the story of the change. Where the line goes up from left to right, the quantity is rising; where it goes down, the quantity is falling; and where it stays level, the quantity is steady. A temperature that climbs from the early morning to the middle of the afternoon shows as a line rising to a high point. The steeper the line, the faster the change, so a sharp climb means a quick rise and a gentle slope a slow one. Describing a section of a line graph means saying whether it rises, falls or stays the same, and how steeply.
Finding the highest and lowest
A line graph makes the highest and lowest values easy to find. The highest point of the line marks when the quantity was greatest, and the lowest point when it was least. On a daily temperature graph, the peak of the line shows the warmest time of day, often in the early afternoon, and the lowest point the coldest, often in the early morning. Locating these points, and reading the time and value at each, answers many of the questions a line graph is drawn to address.
The relationship the graph shows
Looking at the whole line, rather than single points, reveals the relationship it represents. A line that rises to a peak and then falls shows a quantity that increased for a while and then decreased, like a temperature that warmed through the morning and cooled in the evening. The overall pattern, whether it climbs steadily, rises and falls, or levels off, is the relationship between time and the quantity. Discussing this relationship in words is what turns a picture of dots and lines into a description of what actually happened.
Drawing conclusions
From the relationship a graph shows, sensible conclusions can be drawn. A temperature graph that peaks in the early afternoon supports the conclusion that the day was warmest then and coolest around dawn. Conclusions must follow from what the graph actually shows, not from guesses: the graph gives evidence for some statements and not others. Reading carefully also means noticing the scale and the labels, since a conclusion is only as trustworthy as the graph it comes from. Good conclusions stay close to the data.
Reading change with care
Interpreting a line graph means moving from single values to the whole story: read a value by tracing time up to the line, describe each section as rising, falling or steady, find the highest and lowest points, see the overall relationship, and draw conclusions that the data support. With these habits a child can read a temperature chart, a growth graph or any record of change over time, describe what it shows, and say what can fairly be concluded from it.