Very simple example:
CloudDeploy[FormFunction[{"first"->"Number","second"->"Number"}, Plot[1+3*x,{x,0,3}]&,
AppearanceRules -> <| "Title"->"linear function", "Description"-> "two numbers"|>]]
This gives as a result a website with
Graphics[ .... ]
but no plot.
Without FormFunction
CloudDeploy[Plot[Sin[1+2 x],{x,0,6}]]
works as expected.