Maybe something like this? (SUPER UNTESTED!) [EXTRA COMMA FOUND AND REMOVED]

1. float map( float value, float istart, float istop, float ostart, float omid, float ostop){
2.   float imid = ((istop + istart)/2.0);
3.   if( value < imid ) return( map( value, istart, imid, ostart, omid ) );
4.   return( map( value, imid, istop, omid, ostop ) );
5. }

Smart! Looks like it works, once removed the extra comma in the parameter list;

1. float mapm(float value, float istart, float istop, float ostart, float omid, float ostop) {
2.   float imid = (istop + istart) / 2.0;
3.   if (value < imid) return map(value, istart, imid, ostart, omid);
4.   return map(value, imid, istop, omid, ostop);
5. }
6.  
7. void setup()
8. {
9.   size(800, 200);
10.   stroke(255);
11.   for (int i = 0; i < width; i++)
12.   {
13.     float r = mapm(i, 0, width, 0, 100, 20);
14.     point(i, r);
15.   }
16. }

cool thanks

A variant could define at which % of the range the mid value is... (instead of the default 50% we have here).

How to create more a curve effect?

The beauty of map() is that it's a line (or two lines if you use the above function), not a curve.
If you want a curve, simply run your basic mapped line through some other functions. Maybe sin(). Maybe something that squares it. It really depends on what sort of a curve you want!

yeah i just thought of using sin today in class. I'm kinda new to understanding sin and cos.