Function symlog

docs/app/js/sanddance-app.js:106682–106682

(c)

const exp = (sign)=>(x)=>sign * Math.exp(x);
const log = (sign)=>(x)=>Math.log(sign * x);
const symlog = (c)=>(x)=>Math.sign(x) * Math.log1p(Math.abs(x / c));
const symexp = (c)=>(x)=>Math.sign(x) * Math.expm1(Math.abs(x)) * c;
const pow = (exponent)=>(x)=>x < 0 ? -Math.pow(-x, exponent) : Math.pow(x, exponent);
function pan(domain, delta, lift, ground) {