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After writing a while I got unsure about using чтоб instead of чтобы. I checked and found чтоб would be colloquial for чтобы.
However, the ngram indicates a slight rise of using чтоб in literature since the last century.
Is it considered normal using чтоб in written form or is it uncommon somehow?
It is common, just not formal. Consider these: "I own no horse"/"I have no horse"/"I haven't got a horse"/"I got no horse"/"Ain't got no horse". The first is strikingly unnatural and formal, the last is way too informal for most uses.
"Чтоб" is slightly stylistically colored as an informal/old variant, and, of course it is freely used in poetry to keep meter right. It does not mostly sound rude in forums, it sound OK in literature (especially when writing "informal" dialogues). But it is a bad choice if you are writing an essay or some other piece of formal writing (a scientific paper, a legal document etc.)
Remember also that "Чтоб ты сдох/лопнул" is always like that. These type of wishes are not normally expressed "formally" with "чтобы"
Otherwise... For instance, I feel that I seldom ever use "чтоб". There is just no need to (except for "чтоб ты сдох"), and besides, sound much more fluid in speech.
To refer to the part of the question about the historical written usage patterns - one might hypothesize that "чтоб" has seen a rise in usage frequency in the 20th century for three reasons:
before the 20th century the word "чтоб" has been mostly used in its archaic/poetic/high form, i.e. as a direct synonym of "чтобы". This was probably limited to the (now classical) poetry and literature pieces.
in the 20th century, the archaic use was mostly abandoned, with the exception of purposeful stylistic mimicry.
however, the colloquial/pejorative context became much more commonly used in written form, with the liberalization of the language (note how the usage rises sharply right after the October Revolution years in the 1917-1920).
