The partogram is the graphical record of labor in time. The partogram begins at the moment the diagnosis of labor is confirmed by your careprovider. The time prior to the diagnosis is not taken into account.
If the first exam reveals 4cm dilatatation the partogram begins at 4cm on the vertical axis, which corresponds with zero hour on the horizontal axis. The partogram is a prospective tool, not an optional graph to be made in retrospect when your labor has stalled long enough.
This way it is quite visible for everyone if labor is progressing normally. Although simplicity is the key here, still not every doctor or midwife uses this handy tool.
The use of the partogram is based on this book. Proactive Support of Labor is written for professionals such as midwives and obstetricians. It gives a comprehensive explanation of labor and birth, what is normal and what not, and how to approach it professionally in an Evidence Based way.