[racket] improving image quality

Hi,

I'm learning some scribble, along with adding some images from plot.

This works nicely, but i'm trying to work out how to improve  the image
quality in the resulting PDF. (the html output is fine.)

is there any way to make my images sharper and clearer in PDF?

kind regards,

Stephen

----

#lang scribble/base

@(require plot racket/math)

@title{Math 135 Assignment 1}
@section{First Test}

@(define ((deriv f) x)
  (/ (- (f x) (f (- x 0.001))) 0.001))
@(define (thrice f) (lambda (x) (f (f (f x)))))

@(define (x^2 x) (* x x))


@(parameterize ([plot-width    150]
                 [plot-height  150]
                 [plot-x-label  "xaxis"]
                 [plot-y-label  #f])
    (list (plot (function  x^2 (- pi) pi))))
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