An example of the special case where all the data points line dead on a line and the
uncertainties are too small to show as error bars. Determine two widely spaced
locations on the best fit by selecting an x value and reading the y value with a
reading uncertainty from the graph scale. Do not use data points or data values
that where used to plot the graph.

slope

=

\[\frac{(174 \pm 1) - (118 \pm 1) c}{(110-40)s}\]

=

\[\frac{56 \pm 2}{70}\]

=

\[\frac{56c \pm 3.6\%}{70s}\]

=

\[0.800c/s \pm 3.6\%\]

=

\[0.800 \pm 0.029 c/s\]

It is important to emphasize that the slope with its uncertainty is
calculated from the graph and not from data used to plot the graph.