This post follows the previous basic scatterplot with ggplot2. It shows the kind of customization you can apply to circles thanks to the geom_point() options:

• color: the stroke color, the circle outline
• stroke: the stroke width
• fill: color of the circle inner part
• shape: shape of the marker. See list in the ggplot2 section
• alpha: circle transparency, [0->1], 0 is fully transparent
• color: the stroke color, the circle outline
• size: circle size

Note: these options will be uniform among markers if you put it in the geom_point() call. You can also map them to a variable if put inside the aes() part of the code.

Note that applying the theme_ipsum of the hrbrthemes package is always a good option.