The plot area refers to the location of the chart that displays the actual data represented by lines, bars, columns, dots etc. You can easily customize this area, both in size and appearance.
Plot area size
To change the plot area size simply click on it until it is selected, you know it is selected when you see dots around the plot area. The dots around the chart are sizing handles that allow you to resize the plot area inside the chart area.
Click and hold with left mouse button on one of these dots, then drag with mouse to change the size of the plot area, however, you can't make it larger than the chart area.
Customize the plot area
To display the plot area settings simply double-click with left mouse button on the plot area to open the settings pane.
Click on the arrow next to "Fill" to expand and see all settings. "No fill" makes the plot area transparent.
"Solid fill" allows you to use a specific color as a background color, the image to the right shows a black plot area. There is also an option to change the transparency.
"Gradient fill" lets you create a background gradient, there are a plethora of options if you select this.
"Picture or texture fill" lets you pick a background image on your hard drive or online, you can also use a clipart picture.
"Pattern fill" lets you pick a pattern.
The "Automatic" option lets you pick a color if you are not happy with the default one.
The border settings allows you to specify a border around the plot area. The "Solid line" gives you plenty of options to customize the line, color, transparency, width etc.
A "Gradient line" lets you specify many options like type, direction, angle, color, position, etc.
The "Automatic" setting is more or less like the "Solid line" setting.