why would i use a bar graph to show the miles of coastline of westernmost california coasts
Your original question stated that you wanted a graph to show the miles of coastline for California counties. You'll probably have data for a dozen or so counties, each with a different number of miles of coastline. A bar graph is the best to show these differing amounts.
You could also use a circle or pie graph. The entire circle would represent California's total coastline miles. Each county would have a fraction of that circle representing the percentage of coastline each claim.