5.1.1.1 Regular floating point coordinates

Selection of the Cartesian linear transformation with regular floating point coordinates will result in a simple linear scaling $u' = au + b$ of the input coordinates. The projection is defined by stating

$\bullet$ scale in inches/unit (-Jx) or axis length in inches (-JX)

If the y-scale or y-axis length is different from that of the x-axis (which is most often the case), separate the two scales (or lengths) by a slash, e.g., -Jx0.1i/0.5i or -JX8i/5i. Thus, our $y = \sqrt{x}$ data sets will plot as shown in Figure 5.1.

Figure 5.1: Linear transformation of Cartesian coordinates.

The complete commands given to produce this plot were

psxy -R0/100/0/10 -JX3i/1.5i -Ba20f10g10/a2f1g2WSne -W1p,- -P -K sqrt.d > GMT_linear.ps
psxy -R -JX -St0.075i -Glightgray -W -O sqrt.d10 >> GMT_linear.ps

Normally, the user's x-values will increase to the right and the y-values will increase upwards. It should be noted that in many situations it is desirable to have the direction of positive coordinates be reversed. For example, when plotting depth on the y-axis it makes more sense to have the positive direction downwards. All that is required to reverse the sense of positive direction is to supply a negative scale (or axis length).