[Gethe]

LineMixin is the old way of making lines. Lines are actually a real widget type you can create using

local line = frame:CreateLine(...)

or to modify your xml:

<UI>
    <Frame name="DrawFrame" parent="UIParent" frameStrata="LOW">
        <Size x="300" y="300"/>
        <Anchors>
            <Anchor point="CENTER" />
        </Anchors>
        <Layers>
            <Layer level="BACKGROUND">
                <Texture parentKey="Background" setAllPoints="true">
                    <Color r="0" g="0" b="0" a=".70"/>
                </Texture>
            </Layer>
            <Layer level="OVERLAY" textureSubLevel="7">
                <Line parentKey="Fill" file="Interface\TAXIFRAME\UI-Taxi-Line" thickness="64">
                    <Color r="1" g="0" b="0"/>
                    <StartAnchor relativePoint="CENTER"/>
                    <EndAnchor relativePoint="CENTER" x="-300" y="300"/>
                </Line>
            </Layer>
        </Layers>
    </Frame>
</UI>

The methods are similar to the mixin, so it should be easy to switch to. The main difference is that the Set(Start|End)Point methods can accept relative points and relative frames like the normal SetPoint method.