When there are two axes, we call the horizontal axis the abscissa and call the vertical axis the ordinate . There are many other ways to orient coordinate axes, define the unit lengths along them, and to plot data. You may find graphs with axes pointing to the right and down, or to the left and up, etc. Consider first starting with a mathematical equation and proceeding to draw the graph that corresponds to it. Check the intervening points to ensure that they conform with the equation.
Straight lines have the exactly the same slope at all points along the line.
I replotted a graph showing the variation in density of snow, firn, and ice measured at different depths along a 120 m deep drill core through the Greenland ice sheet. The source from which I took these data suggested that the data fall along three distinct straight lines. Compare the rate of change of density per unit depth for the three different processes. The source from which I took these data argued that the three lines correspond to three distinct processes that operate sequentially as snow transforms to ice. In the first step, snow crystals abrade, become rounded, and to settle into a tight packing.