#TI-80 BASIC, 25 bytes - 25% = 18.75
    PARAM
    2π/ANS->TSTEP
    "COS T->X1ᴛ
    "SIN T->Y1ᴛ
    DISPGRAPH
Assumes all settings are set to the default values.
Run the program like `5:PRGM_POLYGON` (for a pentagon)

It works by drawing a circle with a very low number of steps. For example, a pentagon would have steps of 360/5, or 72 degrees.