Yes, it is a valid English word, and it's not new:
The use of graphist seems to have reached peak popularity around 1800:
Note that it means vaguely the same thing as it does today, though its meaning has been overloaded with technology that now exists.
They were written or engraved on bricks, burnt in the fun, which was
probably the earliest rude tablet of the graphist, though afterwards
he committed his thoughts to the more durable substance of marble,
brass, and copper.
Indian Antiquities: Or Dissertations Relative to the Ancient Geographical... by Thomas Maurice, Inigo Barlow, 1800
This is my favourite use, circa 1860:
In the first place, then, the mere power to represent words to the eye
in written -- made with pen and in -- letters, is but a portion of
this branch of education,--the whole of which is equally essential,
though parts of it are not as frequently used as mere word-writing.
The whole branch has been well termed "GRAPHICS", and embraces the
ability to present to the eye, by means of the pen, pencil, or crayon,
on paper or other surface, letters in combination so as to fork words,
arithmetical figures, the mathematical and other signs and diagrams,
and the forms or natural and artificial objects, so far as can be done
by mere lines. To do all this rapidly, neatly and accurately is to
be a graphist, while to be a good writer of words, is to be but
partly a graphist.
The Pennsylvania School Journal, 1860, Volumes 9-11, pages 130 - 131.