loading state data properties for dcc.Graph don't disappear

[chriddyp]

Currently, the loading state data properties are only updated on render, rather than on plot:

dash-core-components/src/components/Graph.react.js

Lines 252 to 266 in b1932f9

	render() {
	const {className, id, style, loading_state} = this.props;
	return (
	<div
	key={id}
	id={id}
	data-dash-is-loading={
	(loading_state && loading_state.is_loading) || undefined
	}
	style={style}
	className={className}
	/>
	);
	}

Since shouldComponentUpdate returns false if the the ID doesn't change, the data properties don't change once they've been added.

dash-core-components/src/components/Graph.react.js

Lines 215 to 220 in b1932f9

	shouldComponentUpdate(nextProps) {
	return (
	this.props.id !== nextProps.id ||
	JSON.stringify(this.props.style) !== JSON.stringify(nextProps.style)
	);
	}

This fixed with two improvements:

1. Update shouldComponentUpdate with a check for loading_state, letting the render re-render with the new data properties
2. Incorporate the plot command so that the loading state is updated once the chart finishes drawing.

1 would fix the bug, 2 would be a slight improvement.

[ohaibbq]

This is resolved in #837