There are several possible answers:
The line is actually meant to be a curved line and not a straight line.
The relationship between the two variables is not a linear function.
Discovering this can be one of the main reasons for drawing the graph.
The data points are empirical measurements (from an experiment, for example)
and there are errors associated with the observations. The errors may be systematic
or random.
You have made mistakes - either in recording the data or in plotting them.
nonlinear graph.
linear graph
a straight line
A Linear graph
In a line graph, the data points represented are typically called "data values" or "data points." The graph displays these values along two axes: the x-axis (horizontal) usually represents the independent variable, while the y-axis (vertical) represents the dependent variable. The line connecting the points illustrates trends or changes in the data over time or another continuous variable.
nonlinear graph.
nonlinear graph
A non-linear graph.
a line of best fit
There is no specific name. It could be a non-linear graph, or a scatter graph.
a line of best fit
a line of best fit
a line of best fit
The definition is any chart where one or more points lie off the straight line joining the other points.
linear graph
a straight line
a straight line