function coerce(attr, dflt) {
        return Lib.coerce(transformIn, transformOut, exports.attributes, attr, dflt);
    }
Supply transform attributes defaults

@param {object} transformIn
 object linked to trace.transforms[i] with 'type' set to exports.name
@param {object} traceOut
 the _fullData trace this transform applies to
@param {object} layout
 the plot's (not-so-full) layout
@param {object} traceIn
 the input data trace this transform applies to

@return {object} transformOut
 copy of transformIn that contains attribute defaults