<script type="text/javascript" language="javascript">
        function Cover(bottomm, topp, ignoreSize) { // это координаты анимации
            var location = Sys.UI.DomElement.getLocation(bottomm);
            topp.style.position = 'absolute';
            topp.style.top =  location.y+ 'px';
            topp.style.left = location.x + 'px';
                      
            if (!ignoreSize) {
                top.style.height = bottom.offsetHeight + 'px';
                top.style.width = bottom.offsetWidth + 'px';
            }
            getpar();
        }

        function getpar() //здесь пытаюсь в скрытое поле передать нужный мне параметр.
        {
          document.getElementById("HiddenFieldPar").value = 254; 
        }
        </script

 <asp:Label ID="Label254" runat="server" Text="254"   OnClientClick="return false;"
        style =" position:absolute; top:222px; left:40px; width:24px; height:13px; background-color: transparent; color:White;  font-family:Arial; font-size:10pt" 
            ClientIDMode="Static" ></asp:Label>

        <asp:Panel ID="pnl254" runat="server" CssClass="pnl" ClientIDMode="Static">
                <div  id="btnCloseParent254" style="float: right;"><asp:LinkButton ID="btnClose254" runat="server" OnClientClick="return false;">Закрыть</asp:LinkButton></div>
                   
            <asp:Chart ID="Chart254" runat="server" DataSourceID="SqlDataSourceGraf" 
                Height="400px" Width="546px" BorderlineColor="Transparent" 
                BorderlineDashStyle="Solid">
                <Series>
                    <asp:Series Name="Series1" ChartType="Line" XValueMember="D_START" 
                        YValueMembers="VAL" BackSecondaryColor="Blue" MarkerColor="Blue">
                        <SmartLabelStyle CalloutLineColor="Indigo" IsMarkerOverlappingAllowed="True" />
                    </asp:Series>
                </Series>
                <ChartAreas>
                    <asp:ChartArea Name="ChartArea1" BackColor="White" 
                        IsSameFontSizeForAllAxes="True" BorderColor="White" BorderDashStyle="Dash">
                        <AxisY TextOrientation="Rotated270" Title="Параметр" TitleAlignment="Far" 
                            TitleForeColor="Gray" Crossing="Min" Enabled="True" IsLabelAutoFit="False" 
                            IsMarginVisible="False" IsMarksNextToAxis="False">
                            <MinorGrid Enabled="True" LineColor="DarkGray" />
                            <LabelStyle Interval="Auto" IntervalOffset="Auto" IntervalOffsetType="Auto" />
                        </AxisY>
                        <AxisX IsLabelAutoFit="False" 
                            TextOrientation="Horizontal" Title="Время" TitleAlignment="Far" 
                            TitleForeColor="Gray" Crossing="Min" Enabled="True" 
                            IsMarginVisible="False" IsMarksNextToAxis="False">
                            <MinorGrid LineColor="DarkGray" Enabled="True" />
                            <LabelStyle Angle="-30" Font="Microsoft Sans Serif, 8.25pt" 
                                Format="dd.MM.yyyy HH:mm" Interval="Auto" IntervalType="Minutes" />
                        </AxisX>
                        <InnerPlotPosition Height="71.89411" Width="80.29147" X="16" Y="6" />
                    </asp:ChartArea>
                </ChartAreas>
                <Titles>
                    <asp:Title Name="Title1" Text="Изменение значений параметров">
                    </asp:Title>
                </Titles>
            </asp:Chart>
            
 
        </asp:Panel>
            <asp:AnimationExtender ID="OpenAnimation254" runat="server"  Enabled="True" 
            TargetControlID="Label254" >
               <Animations>
            <OnClick>
               <Sequence AnimationTarget="pnl254">
               <ScriptAction Script="Cover($get('Label254'), $get('pnl254'), true);" />
               <StyleAction AnimationTarget="pnl254" Attribute="display" Value="block"/>
               <FadeIn AnimationTarget="pnl254" Duration=".2"/>
               <Resize Width="550" Height="400" />
             </Sequence>
            </OnClick>
            </Animations>
            </asp:AnimationExtender>
            <asp:AnimationExtender id="CloseAnimation254" runat="server" 
            TargetControlID="btnClose254" >
            <Animations>
                <OnClick>
                   <Sequence AnimationTarget="pnl254">
                   <StyleAction Attribute="overflow" Value="hidden"/>
                     <Parallel Duration=".3" Fps="15">
                            <Scale ScaleFactor="0.05" Center="true" ScaleFont="true" FontUnit="px" />
                            <FadeOut />
                     </Parallel>
                        <StyleAction Attribute="display" Value="none"/>
                        <StyleAction Attribute="width" Value="100px"/>
                        <StyleAction Attribute="height" Value=""/>
                        <StyleAction Attribute="fontSize" Value="12px"/>
                      </Sequence>
                   
                </OnClick>
                <OnMouseOver>
                    <Color Duration=".2" PropertyKey="color" StartValue="#FFFFFF" EndValue="#FF0000" />
                </OnMouseOver>
                <OnMouseOut>
                    <Color Duration=".2" PropertyKey="color" StartValue="#FF0000" EndValue="#FFFFFF" />
                </OnMouseOut>
             </Animations>

 <asp:HiddenField ID="HiddenFieldPar" runat="server" ClientIDMode="Static" 
       </asp:AnimationExtender>

  protected void Chart254_Init(object sender, EventArgs e)
    {
        HiddenFieldPar.Value = "254";
    }