controlP5 is just a library and I assume does not remove you from using regular Processing code. So, couldn't you just check if the mouse is in the height and width of the bar and then pick an x,y point in that area (or actually anywhere you want) to display the text?

text("text here", x, y);

1. import controlP5.*;
2. 
   
3. ControlP5 cp5;
4. Chart myChart;
5. 
   
6. void setup() {
7.   size(400, 700);
8. 
   
9.   cp5 = new ControlP5(this);
10.   myChart = cp5.addChart("hello")
11.                .setPosition(50, 50)
12.                .setSize(200, 200)
13.                .setRange(0, 10)
14.                .setView(Chart.BAR)
15.                ;
16. 
    
17.   myChart.getColor().setBackground(color(255, 100));
18.   myChart.addDataSet("world");
19.   myChart.setColors("world", color(255,0,255) , color(255,0,0) );
20.   myChart.setData("world", new float[] { 1,2,3,4});
21. 
    
22. }
23. 
    
24. 
    
25. void draw() {
26.   background(0);
27.   
28.   if(cp5.isMouseOver( myChart )) {
29.     String set = "world";
30.     int s = myChart.getDataSet( set ).size();
31.     int n = int( constrain( map( myChart.getPointer().x() , 0 , myChart.getWidth() , 0 , s ) , 0 , s ) ) ;
32.     float value =  myChart.getData( set , n ).getValue();
33.     println(String.format( "value for dataset %s : item %d = %.2f", set , n , value ) );
34.   }
35. }