Add gnuplot syntax

closes #1431

tests/syntax-tests/source/gnuplot/test.gp

@@ -0,0 +1,19 @@
+set terminal pngcairo enhanced
+set output "/tmp/polynomial.png"
+
+set grid
+
+set xrange [-5:5]
+set yrange [-5:10]
+
+set samples 10000
+
+set key bottom right
+
+f(x) = 1.0 / 14.0 * ((x+4) * (x+1) * (x-1) * (x-3)) + 0.5
+
+plot \
+    f(x) title "polynomial of degree 4" \
+    with lines \
+    linewidth 2 \
+    linetype rgb '#0077ff'