Answer:
False.
Explanation:
Time-series graph is a graph that relates something to the change of time, then the time is the single variable in this case.
Bar graphs relate a given quantity to a given variable, an example of this can be the population of a given city (represented by the height of each bar) for each different year (which is the single variable in this case)
Pie-chart is a circular graph with defined sections that represent a given proportion. This kind of graph is used to represent percentages, like in the case of a pie chart that describes the number of men and women in a city. (one section of the circle represents the percentage of women and the other section of the circle represents the percentage of men). In this type of graph, there are no variables, so this is not a single variable graph.
Then the statement:
"Examples of graphs of a single variable include pie charts, bar graphs, and time-series graphs."
Is false.