time series graph is a line graph of repeated measurements taken over regular time intervals.

Time is always shown on the horizontal axis.

On time series graphs data points are drawn at regular intervals and the points joined, usually with straight lines.

Time series graphs help to show trends or patterns.

e.g. Polar Cream Ltd, an ice cream company, shows its sales over the past three years, taken every three months, on the time series graph below. (Quarter 1 is for January, February and March)


From the graph it can be seen that ice cream sales are seasonal (high in summer-low in winter).
There would also seem to be a trend towards higher sales each year.