想用RadioGroup做底部导航页，如何在上面加一条线？

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"

     android:layout_width="match_parent"

    android:layout_height="match_parent">





    <RadioGroup

        android:layout_width="match_parent"

        android:layout_height="wrap_content"

        android:layout_alignParentBottom="true"

        android:orientation="horizontal"

        android:id="@+id/radiogroup">

        <RadioButton

            android:layout_width="wrap_content"

            android:layout_height="wrap_content"

            android:layout_weight="1"

            android:drawableTop="@drawable/radiobutton_home_selector"

            android:textColor="@drawable/radiobutoon_home_text_selector"

            android:layout_marginLeft="20dp"

            android:textSize="15sp"

            android:gravity="center"

            android:text="@string/home"



            android:button="@null"/>





        <RadioButton

            android:layout_width="wrap_content"

            android:layout_height="wrap_content"

            android:layout_weight="1"

            android:drawableTop="@drawable/raidobutton_girdviw_selector"

            android:textColor="@drawable/radiobutton_girdview_text_selector"

            android:text="@string/girdview"

            android:button="@null"

            android:gravity="center"

            android:textSize="15sp"

            />



        <RadioButton

            android:layout_width="wrap_content"

            android:layout_height="wrap_content"

            android:layout_weight="1"

            android:drawableTop="@drawable/radionbutton_internet_selector"

            android:textColor="@drawable/radiobutton_internet_text_selector"

            android:text="@string/internet"

            android:button="@null"

            android:gravity="center"

            android:textSize="15sp"

            />



    </RadioGroup>



    <ImageView

        android:layout_width="match_parent"

        android:layout_height="2dp"

        android:id="@+id/imageView"

        android:background="@color/colorAccent"

        android:layout_above="@+id/radiogroup"

        android:layout_marginBottom="10dp"

        android:layout_centerHorizontal="true" />





</RelativeLayout>