MatrixPlot Styling

Question

I like to build a colour sequence from a list. Each value has a corresponding colour. I used MatrixPlot to do that. I have two question concerning MatrixPlot styling.

Code:

a = RandomInteger[{1, 4}, {50}];

MatrixPlot[{a},
 ColorRules -> {1 -> Red, 2 -> Blue, 3 -> Green, 4 -> Gray},
 Frame -> True,
 Mesh -> True,
 MeshStyle -> Black,
 FrameTicks -> {{None, None}, {Table[i, {i, Length[a]}], None}}] 

Question 1 :

I want to add my own labels on the ticks. For example at position 1 should stand a 2. If I use a list from running from 2 to 51, the labeling starts at the second position.

Question 2 :

I wonder if there is a way to add something to the plot. At the border of 9 and 10 for example I'd like to add a vertical line with text box.

Answer 1

For the tick labels you can use Table[{i, 1 + i}, {i, 1, Length[a]}], and to add a line and text you can use Epilog:

MatrixPlot[{a}, ColorRules -> {1 -> Red, 2 -> Blue, 3 -> Green, 4 -> Gray}, 
 Frame -> True, Mesh -> True, MeshStyle -> Black, 
 FrameTicks -> {{None, None}, {Table[{i, 1 + i}, {i, 1, Length[a]}], None}},
 ImageSize -> 900,  PlotRangePadding -> {{1, 1}, {1, 2}},
 Epilog -> {Thickness[.005], Black, Line[{{8, -.5}, {8, 1.5}}], 
   Text[Style["flag", 14, Red, "Panel"], {8., 2.}]}]