Add description of gas pressure bar graph to user manual

The gas pressure bar graph that has recently been added to the
Information Box on the dive profile is described here.

Signed-off-by: willem ferguson <willemferguson@zoology.up.ac.za>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

Documentation/user-manual.txt

@@ -1811,6 +1811,67 @@ air diving at a depth equalling the END.
 
 Figure (*B*) above shows an information box with a nearly complete set of data.
 
+==== The Gas Pressure Bar Graph
+
+On the left of the *Information Box* is a vertical bar graph indicating the
+pressures of the nitrogen (and other inert gases, e.g. helium, if applicable) that the diver
+was inhaling _at a particular instant during the dive_, indicated by the position
+of the cursor on the *Dive Profile*. The drawing on the left below indicates the
+meaning of the different parts of the Gas Pressure Bar Graph.
+
+image::images/GasPressureBarGraph.jpg["FIGURE:Gas Pressure bar Graph",align="center"]
+
+- The light green area indicates the total gas, with the top margin of the light green
+area indicating the total gas pressure inhaled by the diver and measured from the bottom
+of the graph to the top of the light green area. This pressure has a _relative_ value in the graph
+and does not indicate absolute pressure.
+
+- The horizontal black line underneath the light green margin indicates the equilibrium pressure
+of the inert gases inhaled by the diver, usually nitrogen. In
+the case of trimix, it is the pressures of nitrogen and helium combined. In this example,
+the user is diving with EAN32, so the inert gas pressure is 68% of the distance from the
+bottom of the graph to the total gas pressure value.
+
+- The dark green area at the bottom of the graph represents the pressures of inert gas in each
+of the 16 tissue compartments, following the Bühlmann algorithm, the fast tissues being on the
+left hand side.
+
+- The top black horizontal line indicates the the gradient factor that applies to the depth of
+the diver at the particular point on the *Dive Profile*. The appropriate gradient factor is an
+interpolation between the FGLow and GFHigh values specified in the Graph tab of the *Preferences
+Panel* of *Subsurface*.
+
+- The bottom margin of the red area in the graph indicates the Bühlman-derived M-value, that is the
+pressure value of inert gases at which bubble formation is expected to be severe, resulting
+in decompression sickness.
+
+These five values are indicated on the left in the graph above. The way the Gas Pressure Bar Graph changes
+during a dive is indicated on the right hand side of the above figure for a diver using EAN32.
+
+- Graph *A* indicates the situation at the start of a dive with diver at the surface. The pressures in all
+the tissue compartments are still at the quilibrium pressure because no diving has taken place.
+
+- Graph *B* indicates the situation after a descent to 30 meters. Few of the tissue compartments have had
+time to respond to the descent, their gas pressures being far below the equilibrium gas pressure.
+
+- Graph *C* represents the pressures after 30 minutes at 30 m. The fast compartments have attained
+equilibrium (i.e. they have reached the hight of the black line indicating the equilibrium pressure). The
+slower compartments (towards the right) have not reached equilibrium and are in the process of slowly
+increasing in pressure.
+
+- Graph *D* shows the pressures after ascent to a depth of 4.5 meters. Since, during ascent, the total
+inhaled gas pressure has decreased strongly from 4 bar to 1.45 bar, the pressures in the different tissue
+compartments now exceed that of the total gas pressure and approaches the gradient factor value (i.e.
+the top black horizontal line). Further ascent will result in exceeding the gradient
+factor value (GFHigh), endangering the diver.
+
+- Graph *E* indicates the situation after remaining at 4.5 meters for 10 minutes. The fast compartments
+have decreased in pressure. As expected, the pressures in the slow compartments have not changed much.
+The pressures in the fast compartments do not approach the GFHigh value any more and the diver is safer
+than in the situation indicated in graph *D*.
+
+
+
 == Organising the logbook (Manipulating groups of dives)
 
 === The Dive List context menu