data = {
if( !data_cache[datafile] ) {
console.log('fetching ', datafile);
let new_data = await fetch( endpoint )
.then((response) => response.json())
.then( json => json.filter( d => d.location ) )
mutable data_cache[datafile] = new_data;
return new_data
} else {
console.log('from cache', datafile)
return data_cache[datafile]
}
}