

This diagram does not physically exist, it would be impossible to maintain such temperature and pressure gradient. What it helps with is identifying which state water is in under given conditions. Find the temperature you’re interested in on the x-axis and the pressure you’re interested in on the y-axis, project these values at right angles to find a point in the chart and the color at that point is the state water is in for the specific conditions.



















If you have a meter on the wall that says “height”, height exists outside the meter too. Height is a property of every point in the room. For any point, you can follow a horizontal line to the corresponding point on the meter, read the number, and that’s the height at that spot. Also, every point on a horizontal plane will have the same height: all points on the kitchen counter will have a height of about 83 cm, every point on the ceiling a height of about 230 cm. The meter is basically an axis and you’re using it to measure how high various points throughout the room are.
Then people realized you can do it in 2 or 3 dimensions and represent data.