When you use the “maxwidth” or “maxheight” keys context ignores

the current bodyfont and also the “bodyfont” key to fit the chart on the page.

What is the reason for this? 

Try your example without \startTEXpage/\stopTEXpage and you see that the

chart won’t fit on the page when you set the fontsize to 24pt. To keep the width

of the chart within the allowed width (\textwidth) context reduce the fontsize

until the chart fits.