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Casio made the first graphing calculator, the fx-7000G. See US Patent 4794553 (note that the foreign priority date of May 24, 1985, is 1 1/2 years before introduction of the HP-28C).
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I think it was Casio, then followed by the Hp28C by a few months later. I am thinking either a Casio fx-6000 series or the Casio fx-7000.
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It depends on how one defines 'graphing calculator'. If you
mean a large graphical screen with medium to large resolution, you are probably right.
But if it is defined to mean a handheld calculator with
an LCD dot-matrix display where the dots are individually addressable, you could consider the HP-71B, which has
a one-line, 132x8 display, and the Sharp PC-1500 (aka TRS 80
PC-2) which has a 156x7 display, though the Sharp PC-1350,
a later model (though still vintage) qualifies better as it has a much larger
150x32 graphical display while being smaller and much
lighter than either the HP-71B or the Sharp PC-1500.
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You have to go way back -- I strongly recommend "The Pencil -- A History of Design and Circumstance" (or something like that) by Henry Petroski . . .
Ohhh! You asked about graphing calculators . . .
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The HP42S (c.1987) can graph on its two line display. HP even included a program in the user's manual to do it.