A straight-line graph looks like this. In mathematics, it denotes a linear function. We can tell from this graph that the data trend is changing at a steady rate across time. A straight-line graph can go diagonally upwards or downwards in any direction. A straight-line graph is the most reliable tool for anticipating data trends. It's simple to make it look like a line graph with just one data set, and you can compare two straight-line data sets with it.