Custom palettes need at least two elements, not at least one.

Change the error check + documentation.

Fixes #99

Author: SiegeLordEx

gnuplot/src/axes_common.rs

@@ -1395,9 +1395,9 @@ impl AxesCommonData
 			}
 			Custom(ref entries) =>
 			{
-				if entries.is_empty()
+				if entries.len() < 2
 				{
-					panic!("Need at least 1 element in a custom palette");
+					panic!("Need at least 2 elements in a custom palette");
 				}
 				write!(w, "set palette defined (");
 

gnuplot/src/options.rs

@@ -421,7 +421,7 @@ pub enum PaletteType<T>
 	/// Use a cube helix palette, with a certain start (in radians), cycles, saturation and gamma.
 	CubeHelix(f32, f32, f32, f32),
 	/// A custom palette
-	/// is specified by a sequence of 4-tuples (with at least one element). The first
+	/// is specified by a sequence of 4-tuples (with at least two elements). The first
 	/// element is the grayscale value that is mapped to the remaining three elements
 	/// which specify the red, green and blue components of the color.
 	/// The grayscale values must be non-decreasing. All values must range from 0 to 1.