Dégradé approche texte

Here's my version, using tracking. It can do left- and right- justified text. It's a bit messy and have only tested on basic text frames but hopefully gives you something else to try, as well as femkeblanco's script. Note: after applying the tracking, you can copy the text out and into another shape text frame to achieve the curve in your example.

// these are just examples that modify the tracking you can make your own or change these
var exampleAdjustment1 = function (n) { return n + 30 };
var exampleAdjustment2 = function (n) { return (n + 10) * 1.1 };


// call the spaceOut function on the selected item (must be a TextFrame)
spaceOut(selection[0], exampleAdjustment1);



function spaceOut(txtFrame, adjustment, baseTrackAmount) {
    if (!txtFrame) return false;
    if (txtFrame.typename != 'TextFrame') throw 'spaceOut requires TextFrame item';
    txtFrame.textRanges[0].paragraphAttributes.hyphenate = false;
    if (baseTrackAmount == undefined) baseTrackAmount = 0;

    // make an array of lines, containing an array of characters
    var _lines = [];
    for (var i = txtFrame.story.lines.length - 1; i >= 0; i--) {
        // first add line break after the line so it doesn't wrap
        txtFrame.story.lines[i].contents += '\n';

        _lines[i] = [];
        for (var j = 0; j < txtFrame.story.lines[i].characters.length; j++) {
            _lines[i][j] = txtFrame.story.lines[i].characters[j];
        }
    }

    // get font size, used for calculating text box width expansion
    var fontSize = _lines[0][0].characterAttributes.size;

    // get justification
    var isJustifiedLeft = txtFrame.paragraphs[0].paragraphAttributes.justification == Justification.LEFT;

    var start, end, step;
    var longestLineAddedSpace = 0;
    // loop over lines
    for (var i = 0; i < _lines.length; i++) {
        var trackAmount = baseTrackAmount, total = 0;

        // these determine the looping order
        if (isJustifiedLeft) {
            start = 0, end = _lines[i].length, step = 1;
        } else {
            // right justified, start at last character, going backwards
            start = _lines[i].length, end = 0, step = -1;
        }

        // loop over characters
        for (var j = start; j != end; j += step) {
            var ch = _lines[i][j];
            if (!ch) continue; // empty line
            if (ch.length > 1) ch = ch.characters[ch.length - 1]; // had to do this due to unexpected value of j=0, but don't know why!

            // apply tracking
            ch.tracking = trackAmount;

            // keep record of running total for this line
            total += trackAmount;

            // update track amount according to function
            trackAmount = adjustment(trackAmount);
        }
        longestLineAddedSpace = Math.max(longestLineAddedSpace, total)
    }

    // adjust width of text box to accommodate added space
    try {
        addedSpaceInPts = (longestLineAddedSpace / 1000) * fontSize;
        txtFrame.textPath.width += addedSpaceInPts;
        if (!isJustifiedLeft) txtFrame.textPath.left -= addedSpaceInPts;
    } catch (error) {
        $.writeln(error);
    }
}

This is a basic idea

var string1 = selection[0].contents.replace(/ /g, "          ");
selection[0].contents = string1;
var paragraphz = selection[0].paragraphs;
for (var i = 0; i < paragraphz.length; i++) {
    var wordz = paragraphz[i].words;
    for (var j = 0; j < wordz.length; j++) {
        var characterz = wordz[j].characters;
        for (var k = 0, l = characterz.length - 1; l > 0; k += 1 + l, l--) {
            for (var n = 0; n < l; n++) {
                characterz.add(" ", characterz[k]);
            }
        }
    }
}