By drawing a line between that point back to the fuel cost lines (the light blue line) we can see how many more items could be transported with the fuel just saved, which would be only 4.927 items. When transporting 100 items 10 tiles, the normal cost will be 176.3 petrol while with a warehouse the cost will be 168.021, for a savings of 8.279 which can be seen in the bottom right corner.
Here there are a bunch of numbers going on, so lets first start in the top right. The lower dark blue line is the savings line (green line minus red line), and the light blue line will be explained below. The green line is fuel costs, and red line is fuel costs with warehouses. This savings when working at this scale may not seem like much, (at 6 tiles you get a savings of 0.5 petrol), and for the most part you would be correct. Because both of these are linear lines, graphing the fuel savings given by warehouses is also linear.
