A rectangular graph paper is a graph paper where the x-axis and the y-axis are divided into equal divisions. Each of these divisions represents a fixed value throughout the x or the y axes (linear scale)

The x-axis and the y-axis may cover different ranges of the graphed variables. The minimum value on the axes may or may not be zero.

The slope of a curve at any point on the curve is the rate of change of the dependent variable (the variable plotted on the Y-axis) relative to the change of the independent variable (the variable plotted on the X-axis) at this particular value of X and Y coordinates.

The curve usually has changing slope at different points. The slope at any particular point on the curve is the slope of the tangent drawn at this point.

A plot of the amount of the drug in the body versus time is a curve with decreasing slope. This means that the rate of change of the amount of the drug in the body (which is the rate of drug elimination) is decreasing with time. The rate of drug elimination is decreasing with time because the amount of the drug in the body is decreasing due to drug elimination.