First of all, overflow
will not work with the chart because charts are rendered into img tags and you can't set overflow to an image.
If I got you correctly I think this would be what you're looking for, try it:
First you have to set the viewport meta tag in your page heading:
<meta name="viewport" content="width=device-width; initial-scale=1.0; maximum-scale=1.0;" />
And in the aspx page:
//The left div you mentioned in the comment
<div id="lefty" style="float:left;width:18vw;height:100vh;border:1px solid #000;"></div>
//The wrapper div of the chart
<div id="wrap" style="width:80vw;height:100vh;overflow:scroll;">
<asp:Chart ID="Chart1" runat="server" BorderlineColor="Black"
BorderlineDashStyle="Solid" BackColor="#B6D6EC" BackGradientStyle="TopBottom"
BackSecondaryColor="White" Height="600px" Width="1800px" Style="height:100%;">
<ChartAreas>
<asp:ChartArea Name="ChartArea1">
</asp:ChartArea>
</ChartAreas>
<Titles>
<asp:Title Alignment="TopCenter" Font="Verdana, 12pt, style=Bold" Name="Title1"
Text="Graph" ForeColor="Blue">
</asp:Title>
</Titles>
</asp:Chart>
</div>
Briefly, what I did is that I set the width and height of the wrapper div as a percentage of the width and height of the viewport width and height and I set the wrapper to scroll the overflown part of the chart.
Hope this helps...