Parameterized Javascript implementation of https://twitter.com/slimedaughter/status/927729872092270592 just for the hell of it.

wiggle.js

/**
 * Return an array of objects representing a triangle wave, with a values and direction for each point.
 * @param {Number} length - The desired length of the array.
 * @param {Number} height - The desired peak value for the wave.
 * @returns {Object[]} - Array of objects like `{wiggle: <Number>, wiggle_down: <Boolean>}`.
 */
function makeWiggle(length, height) {
    const output = []
    let wiggle = 0;
    let wiggle_down = false;
    output.push({wiggle: wiggle, wiggle_down: wiggle_down});
    for (let i = 0; i < length - 1; i++){
        if (wiggle < height && !wiggle_down) {
            wiggle += 1;
        } else if (wiggle >= height && !wiggle_down) {
            wiggle_down = true;
        } else if (wiggle > 0 && wiggle_down) {
            wiggle -= 1;
        } else if (wiggle <= 0) {
            wiggle_down = false;
        }
        output.push({wiggle: wiggle, wiggle_down: wiggle_down});
    }
    return output
}
/**
 * Return a 2D array of strings representing the wiggle defined in `wigglesArray`.
 * @param {array} wigglesArray - Output from `makeWiggle()`.
 * @param {int} height - The max wiggle value from `wigglesArray`
 * @returns {Array.<string[]>}
 */
function prepareWigglesOuput(wigglesArray, height) {
    const graphHeight = height + 1;
    const pixelMap = Array(graphHeight).fill(undefined).map(() => Array(wigglesArray.length).fill('·'));
    for (let i = 0; i < wigglesArray.length; i ++) {
        pixelMap[-1 * (wigglesArray[i].wiggle) + graphHeight - 1][i] = `${wigglesArray[i].wiggle_down ? '↓' : '↑'}`;
    }
    return pixelMap
}

/**
 * Return a string-rendering of a triangle wave.
 * @example
 * renderWiggles(10, 3)
 * // · · · ↑ ↓ · · · · ·
 * // · · ↑ · · ↓ · · · ·
 * // · ↑ · · · · ↓ · · ↑
 * // ↑ · · · · · · ↓ ↑ ·
 * @param {Number} length - The desired length of the wave.
 * @param {Number} height - The desired peak value for the wave.
 * @returns {String}
 */
function renderWiggles(length, height) {
    const wiggles = makeWiggle(length, height);
    const pixels = prepareWigglesOuput(wiggles, height);
    return pixels.map(row => row.join(' ')).join('\n');
}

console.log(renderWiggles(40, 6));