For the time being you can manually fetch the data and pass it to the chart:
		<script>
			function pageLoad() {
				var dataSource = $find('<%= RadClientDataSource1.ClientID %>');
				dataSource.fetch(function (args) {
					var data = args.get_data(),
						chart = $find('<%=RadHtmlChart1.ClientID%>');
					//workaround
					chart.set_dataSource(data);
					chart.repaint();
				});
			}
		</script>
		<telerik:RadClientDataSource ID="RadClientDataSource1" runat="server">
			<DataSource>
				<WebServiceDataSourceSettings>
					<Select Url="spain-electricity.json" DataType="JSON" />
				</WebServiceDataSourceSettings>
			</DataSource>
			<Schema>
				<Model>
					<telerik:ClientDataSourceModelField FieldName="year" DataType="String" />
					<telerik:ClientDataSourceModelField FieldName="solar" DataType="Number" />
				</Model>
			</Schema>
		</telerik:RadClientDataSource>

		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" ClientDataSourceID="RadClientDataSource1">
			<PlotArea>
				<Series>
					<telerik:LineSeries DataFieldY="year" Name="year">
					</telerik:LineSeries>
				</Series>
				<XAxis DataLabelsField="year"></XAxis>
			</PlotArea>
		</telerik:RadHtmlChart>