I am puzzled by the following in your post: "Anyway, there seem to be very few really blue stars. Whereas the graph might lead us to expect more
". Why do you think the curve would lead you to expect more blue stars? It isn't showing you the different proportions of stars, merely the spread of radiation produced by objects of differing temperature.
As to why, with around 75% of all stars being spectral type M, we don't see most stars in the sky as red - *shrugs*
Anyway, to answer your question about green stars...
Looking at the curve again:
Looking at the bottom curve (for 2,000K). For low temperature stars there is practically no (well very very little) radiation being emitted at the high-energy, blue end of the spectrum. The graph rises gently towards the red-end of the spectrum and continues to rise well into the infra-red. So these low-temperature stars are (at best) red in colour (if you can see them).
If you extrapolate the dotted line to higher temperatures and imagine what the curve will look like, at around 4,000-4,200K - the peak of the curve will be on the lower boundary of the visible spectrum. Again, not so much violet and blue light, but far more towards the orange/red part of the spectrum - so the star would again have a reddish-cast (and would probably look orange).
At even higher temperatures, the dotted line still climbs and moves towards the left but by the time the peak is in the middle of the visible band, you have a fair amount of violet and blue, a bit more green and a fair amount of yellow, orange and red - in other words you have a complete spectrum and so the star appears white. This is shown in the picture below - erroneously stating that the surface temperature of the Sun is 6,000K - it is in fact around 5,780 K (5,505 °C).
Continuing the extrapolation of the dotted line - ever higher and still moving to the left at higher temperatures. Imagine what the curve looks like at very high temperatures, where the peak is at the violet end of the spectrum. The right-hand side of the curve drops steeply, not as steeply as the left-hand side but steep enough. Therefore you have a preponderance of blue/violet light - so the star looks blue.
There are no green stars due to the fact that when the peak of the curve is sitting squarely in the middle of the visible spectrum (as it is with the Sun which peaks in the yellow-green part of the spectrum), you have a lot of blue/violet radiation to one side and a lot of yellow/orange/red radiation the other. The distribution is fairly even across the visible spectrum - even though it is centred around green - and because we have a full mix of colours, we see the star as white.
This curve may help to explain the above (for temps up to 6,000K):
So, in summary:
Very cool stars radiate in the infra-red, we can't see them in the visible = brown dwarfs
Cool (M-type) stars radiate in the red and orange. These are red giants (Antares and Betelgeuse) and Red Dwarf stars (Proxima Centauri) with a surface temerature around 3,500K (3,200ºC). M-type stars are the most common with around three-quarters of all stars being type-M. Orange Dwarfs (K-type) such as Alpha Centauri B and Epsilon Indi. Roughly 1 star in 8 is a type K star (12.5%), surface temperatures are below 5,200K.
A slightly hotter star (Yellow Dwarf, G-type) has a surface temperature aroung 5,200-6,200K and the Sun is a prime example along with Alpha Centauri A, Capella and Tau Ceti. The spectral classification of the Sun is actually G2 (each class - O, B, A, F, G, K, M - being split into 10 smaller sub-classes). Around 1 star in every 12 is a G-type star like the Sun.
Hotter than this we have the F and A-type. F-type stars are yellow-white (examples being Canopus, Dubhe B, Polaris and Procyon) with a surface temperature of around 6,000-7,500K. Around 1 star in 30 is a type-F star. A-type stars are white (examples being Sirius, Deneb, Altair and Vega) and have a surface temperature between 7,500K and 10,000K. Only about 1 star in 160 of the main-sequence stars are spectral type A.
Even hotter are B-type stars with a surface temperature of between 10,000K and 33,000K (examples being the brighter stars of the Pleiades, Algol A, Rigel, Spica and VV Cephei B). These stars are bluish-white in colour. Only about 1 star in 800 of the main-sequence stars are B-type stars.
O-type stars are textremely hot with surface temperatures above 33,000K. These stars are also the most massive stars - the Blue Supergiants and they appear, well, blue. Examples are Zeta Orionis, Zeta Puppis, Lambda Orionis, Delta Orionis and Theta Orionis C. Interstingly, these stars output most of their energy in the ultraviolet range. They are the rarest of all main-sequence stars with only about 1 star in three million being O-type stars. Another interesting fact is that these stars shine with a power output of over a million times that our own Sun!
There is a further spectral class - type W - with surface temperatures in excess of 50,000K.
This Wiki article should help. There is a table under the heading "Harvard Spectral Classification" which shows the stellar classes (O, B, A, F, G, K, M) and their associated colours. A mnemonic to remember these (in decreasing-temperature order) is: W
e A F
I hope this is of some interest and answers your question as to why there are no green stars.