I guess that would be a line on which you would record data about the international date line.
The straight line that best fits the data on a coordinate plane is the Line Of Best Fit.
The International Date Line.
a line of best fit
a line of best fit
Because the "best fit" line is usually required to be a straight line, but the data points are not all on one straight line. (If they were, then the best-fit line would be a real no-brainer.)
linear graph
a line of best fit
The International Date Line follows the 180th meridian to prevent dividing countries into two different calendar days. It bends around certain territories and islands to ensure that each country remains within a single day. This results in the International Date Line not being a straight line.
a line of best fit
A Linear graph
to avoid pacific islands so they are in the same timezone