How would you explain the phase diagram of sulphur?

How about this:

A phase diagram is a chart that shows the conditions of pressure and temperature at which distinct phases occur and coexist at equilibrium.

The lines on a phase diagram divide into regions – solid, liquid, and gas.

The phase diagram of sulfur is

The diagram is complicated by the fact that sulfur can exist in two crystalline forms: rhombic and monoclinic.

Let's look first at the four areas:

• Pink — only rhombic sulfur
• Brown — only monoclinic sulfur
• Green — only liquid sulfur
• Blue — gaseous sulfur

The corresponding curves are:

• lower left to ① — the sublimation curve of rhombic ##"S"##: ##"S(rhombic)" ⇌ "S(g)"##
• ① to② — the sublimation curve of monoclinic ##"S"##: ##"S(monoclinic") ⇌"S(g)"##
• ② to upper right — the vapour pressure curve of liquid ##"S"##: ##"S(l)" ⇌ "S(g)"##
• ① to ③ — the transition curve for ##"S(rhombic)" ⇌ "S(monoclinic)"##
• ② to ③ — the melting point curve for ##"S(monoclinic) ⇌ S(l)"##
• ③ to top — the melting point curve for ##"S(rhombic) ⇌ S(l)"##

There are three triple points:

• ① (##"95.4 °C", 1 × 10^"-4"color(white)(l) "atm"##) — rhombic ##"S"## is in equilibrium with monoclinic ##"S"##, and both have the same vapour pressure.
• ② (##"119 °C", 5× 10^-4color(white)(l) "atm"##) — monoclinic ##"S"## melts; this is the triple point for ##"S"_"m" ⇌ "S"_"l" ⇌ "S"_"g"##.
• ③ (##"151 °C, 1288 atm"##) — rhombic, monoclinic, and liquid ##"S"## are at equilibrium.

The critical point — where liquid and gaseous ##"S"## have the same density — is off to the right at ##"1041 °C and 203.3 atm"##.